Gray water tank install

Gray water is any water that does not contain fecal material. There is some debate as to whether urine and kitchen waste qualifies as graywater as it may contain minor amounts of bacteria, but for my purposes the urine from my composting toilet and the drainage for my kitchen sink will go into my graywater tank.

Since my wet bath and sink are on the driver’s side of the vehicle, I wanted to locate the graywater tank on the underside of the vehicle on that side. The maximum size tank I could fit would be about 60 inches long, 12 inches wide, and 9 inches tall. However, cramming a tank that large into that space would interfere with routine maintenance on the Webasto fuel pump, limit access to dropping the main fuel tank, preclude the use of side fittings for drainage on the tank itself, and require flat bar straps instead of angle iron to hang the tank.

I thought I found the perfect roto-molded, light weight 16 gallon tank with dimensions 56” x 10” x 8.5”, but when it arrived, it was only 7 inches wide, with a 10 gallon capacity. The website was wrong, and the tank I wanted did not actually exist. I had already designed my plumbing and fixture locations with this particular tank in mind, so I ended up having a custom tank made from ½” HDPE with the correct dimensions.  The tank was $200 with shipping. The custom tank from was $500 with shipping. That was the cheapest of four bids.

Vintage Trailer has a nice overview of tank design and considerations.

To recap:

  • Any water tank must have a vent at the top to allow water to drain out without creating a suction.  Ideally the vent does not allow anything to slosh out on the road. My tank has a 3/4” vent located at the top of the left hand side.

  • Standard RV tank drain sizes are 3 inch and 1.5 inch. The larger diameters are used because most RV’s have a black water tank AND a gray water tank, and of course the black water tank needs a nice large diameter. The gray water tank is used to flush the drain hose after the black water tank is dumped. The larger diameters also create a flushing effect inside the tank when dumping, as well as reduce the time you spend standing at the pumpout station. Since I only have a gray tank, I could have probably gotten by with a smaller valve. I am using a 1.5 inch NPT fitting, which works with the standard RV gate valves.

  • Gray and black water tanks build up nasty smells you want to prevent backing up through your drain into your interior. You need to create a p-trap - a loop in the fixture drain hose with water sitting in the bottom to prevent the smells from entering the van.

  • Tanks and pipes freeze – plan how to handle this. If you are camping in the winter, you need a tank heater and P-trap heaters. If you’re just storing in the winter, you need a way to drain them.

  • Wastewater tanks are generally sloped for drainage to prevent sediment buildup at the back of the tank.

I am mounting the tank using a 1” angle iron tray bolted through the floor of the van. With a  total weight when full of 170 pounds, it was important to me that the tank stay put in an accident, and not warp the floor in off road adventures.

I am also installing a cable that will allow me to empty it from inside the van, a tank heater pad, and two elbow pads for living in below freezing temperatures.

Dumping the tank

Most information about dumping RV graywater tanks is for those who are spending their time at RV campgrounds. Camping World has a good overview of the standard tank dumping process. But we do not have a black water tank, and most of our time will be boondocking, although we do plan on being at campgrounds with dump stations every week or so for laundry, hot showers, etc.

For those times when we are in a campground, I wanted to be able to dump the graywater tank faster than gravity drain through a garden hose. So I adopted the standard 1.5 inch drain size. The next problem to solve was how to open the valve without having to lay on my back and crawl under the vehicle. I looked at 1.5” electric ball valves, which could conceivably grind their way through small kitchen waste solids, but they were either not rated IP67 for extremely wet locations, frozen locations, or they were really expensive. And most RV parts vendors say that ball valves don't last very long in this application.

I finally decided on the standard RV ABS gate valve approach, with a cable that allows me to open it from the galley area. I can drain the tank from inside or outside the vehicle by reaching through the window and pulling the lever.

Designing P-traps

Most RVs seem to use standard household P-trap's. I didn't want to waste space inside my cabinetry or under the van with these flimsy and troublesome components. I am using marine sanitation hose, and bending a loop in it to form the P-trap just below where it enters the tank. For the galley I just spaced the hole in the van and the fitting the right distance apart. For the bathroom, if needed, I can put a hose clamp at two points along the hose, and run some copper pipe strap between the two points to force the bend where I want it.

Preventing freezing

If the van is unoccupied, we’ll winterize it by draining the tanks. But if we’re living it it, we need to prevent the tank and and hoses with water in them from freezing. I found a 7.25” x 12” RV tank heating pad. It’s encased in 1/8” closed cell foam, easily applied to the bottom of the tank, water and corrosion resistant, and has a thermostat for energy efficiency. It can be trimmed a bit narrower to fit within my tank’s mounting tray. I also used two elbow heaters for the p traps on my drain hoses, and because the elbow heaters don’t have a thermostat, they get too hot for my vinyl hose.  I will install a PWM dimmer switch to reduce the heat level when my panel is finally in. I am not installing a heater for the waste valve. I can either leave it open or use a blow dryer to warm it up if it freezes shut.

Monitoring tank levels

I will be monitoring my fresh, grey, and propane tank levels using the SeeLevel II system. For the grey water tank sensor, I’ll be using an 12” sensor cut to fit.



  • Metal bandsaw for cutting the angle iron

  • 110v welder for welding up the frame

  • Angle grinder and wire brush to clean up the welds

  • Screwgun, step drill, and drillbits to drill holes

  • Acetone, primer, paint, and undercoating

  • Center punch to mark your holes

  • Floor jack to easily raise and lower your tank into position multiple times


  1. Check the tank for fit underneath your vehicle and decide on the final location. Keep in mind clearance for fittings, access to mounting brackets, etc.

  2. I mounted the tank so that the two 1.5” angle cross bars were right up against and behind the tranverse frame members.  In a front end accident, the tank will surge forward, but the brackets will be locked in place up against these members.

  3. Use a sharpie on the frame to mark where the tank brackets will located. 

  4. Cut two 12.5” lengths of 1.5” angle iron.  Test their fit underneath the vehicle, behind the framing members.  Trim away any excess undercoating that would prevent a flush fit. In my case the floor steel lapped over the framing member, creating a slight angle when bolting the bracket flat. That’s ok. We’ll angle the tray bracket to match.

  5. Drill 2 7/16” holes in each cross member – keeping in mind that there will be 2” x 4” x ¼” steel washers on the floor above. So you need to avoid the floor ridges when planning these holes.

  6. Hold the cross members against the floor underneath the vehicle, and center punch where you want to drill the holes for the floor.

  7. I found it easiest to do a small hole first which would then guide the larger drillbit. So I drilled using an 8 inch drill bit first, then a step drill bit.

  8. Temporarily bolt the cross members to the floor.

  9. Cut 4 12” lengths of 1” angle iron.  These will be welded to the 1.5” cross members, and hang down on either side of the tank.

  10. Drill a 3/8” hole 1” from the end of each of these pieces. The tank tray will bolt to these.

  11. Hold 2 of the 12” pieces up against a 1.5” crossmember, and mark on the crossmember where it should be welded. You want one side of the angle to be touching the tank, and the other side of the angle to be touching the crossmember.

  12. Remove the cross members from the floor, and weld the 12 inch pieces. Make sure they are square and parallel, and a bit further apart than the tank itself to account for the thickness of the tray angle iron.

  13. Reattach this upper assembly to the floor, bolting it down reasonably tight so that it is in its final position.

  14. Create the tray for the tank. Cut 2 56” pieces of 1” angle iron at a 45° angle, and two 10” pieces of angle iron also at a 45° angle. Place them on the underside of the tank and tack weld them in place. This ensures a perfect fit.

  15. Remove from the tank and finish welding the joints. Don’t do any welds which will interfere with the tank seating fully in the tray.

  16. Weld a flat bar cross support across the middle of the tray to prevent the center of the angle iron from bowing out over time.

  17. Place the tray on the underside of the tank, and jack into position.

  18. Mark on the tray the angle and location where the vertical angle brackets are hanging down. You will use this to determine where to weld the tray angle brackets that will bolt to the vertical angle brackets.

  19. Remove the tank and the tray from the tank.

  20. Cut 4 4” pieces of 1 inch angle iron.

  21. Weld all four pieces onto the tray at the locations you marked. Keep in mind the orientation of the face of the angle so that it mates correctly with the vertical angle bracket.

  22. Reattach the tray to the tank, and jack the tank into position again.

  23. Check the final fit, and then use a sharpie to mark the locations for the holes in the 4 inch angle iron brackets.

  24. Remove the tank (again!) and drill these four holes.

  25. Using an angle grinder around the corners of all of your metal, clean up your welds, and deburr all surfaces. Sharp corners will come back to haunt you. Use a wire brush to remove any loose mill scale.

  26. Clean with acetone, prime and paint all metal

  27. Attach the 3/4” vent fitting, and then slip on a 4” piece of hose sticking straight up from the tank

  28. Attach the 1” side drain fitting (easier to do this now)

  29. Give the paint a few days to harden, and finally bolt the tank permanently into place

Running the drain hose

I need to do the galley drain hose now, before the wall paneling goes on.

  • I had fiddled with grommets elsewhere on the van, and they were such a hassle I ended up wrapping the hose in Gorilla tape where it passes through sheet metal.

  • I sprayed a bit of Great Stuff foam where the hose passes through the floor to keep out moisture and critters.

  • Pro tip: When working with stiff hose heating it up makes it more flexible, and spraying fittings with soapy water make it easier to slide on.

Installing the drain cable

I also installed the Valterra drain cable. Not much to say, but here are a few tips:

  1. Their warranty is pretty specific about minimum bend radius and maintaining 6” straight at the beginning and the end of the line. That’s why my tank has the drain outlet inboard rather than outboard - to ensure I had sufficient straight run.

  2. I ran a piece of PEX from underneath the vehicle through 4 different pieces of van sheet metal. Then I ran the cable housing inside of this. I did this to protect the cable housing, make it easier to replace the cable housing in the future, and ensure that the cable housing did not exceed the minimum bend radius of 24”.

  3. I sprayed silicon on the cable, housing, and valve before final installation

Still to do…

  • It was too cold today to put the main tank heating pad on.

  • The drain for the shower/toilet will go on when that cabinet goes in.

  • I haven't yet sorted out the best way to hook up a drain hose to the drain fitting.