Monday, September 25, 2023


Quick RV Tech Tip: Why doesn’t my gray water tank drain completely?

Dear Dave,
Since the day I bought my RV the gray tank does not empty completely. It starts off with a good force then either stops draining or goes down to a very slow trickle. I don’t believe it empties completely. —Deborah, 2018 Wildwood DLX 353flfb

Dear Deborah,
Since the gray tank doesn’t get much “debris” that would clog the drain pipe or have toilet paper that doesn’t dissolve, I doubt it’s anything obstructing the flow. The gray tank is mainly the shower drain and a sink or two, depending on the location of them. If your kitchen sink drains into the gray tank, you might have some food or grease in it so I would start by cleaning it thoroughly with Thetford’s Tank Blaster just to make sure.

Remove the shower drain cover and flush out the tank with a garden hose and see how the fluid flows. How do you know the tank is not drained completely? Are you going by the monitor panel inside? This may not be accurate.

One thing I would try is to raise the opposite side of the rig from the drain valve with a floor jack and see if it drains everything out. It’s not uncommon to have a rig weigh more on one side than the other once you load everything in the compartments and it might not be level enough to drain. The RV Safety & Education Foundation has been weighing rigs for more than 30 years. Still, more than 50 percent are over the recommended weight ratings in either GVWR, GAWR, or side to side. Some rigs are more than 1,000 lbs. heavier on one side! And it’s not uncommon for units to settle slightly over time.

Finally, check your roof vent to make sure it is not clogged and creating a vacuum. I would recommend replacing the standard vent with the Siphon 360.


Dave Solberg
Dave Solberg
Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club. He has been in the RV Industry since 1983 and conducts over 15 seminars at RV shows throughout the country.


  1. (former Safari Motor Coach Plant Engineer)
    Try to determine where/how the drain pipe is attached to the tank. There are 3 possibilities, some that may trap liquid in the tank AND cause debris that settles to the bottom to not drain. 1) Ideally, drain connections SHOULD BE on the bottom but rarely are due to ground clearance. 2) The next best thing is for there to be molded ‘low point’ in the tank so a side-placed fitting will be lower than the rest of the bottom of the tank. 3) The worst are tanks that are flat bottomed (or with so little slope they are effectively flat) with no low point. Drains may be connected to molded spigots (male projections) or ‘spin fittings’ which are friction welded to the face of the tank after a hole is cut. A spin fitting needs to be about 3/4″ from the corner (bottom) of the tank. If the tank has a spin fitting in a location that does not include a molded ‘low point’, it will hold water AND accumulate debris.

  2. When I owned a Casita Travel Trailer I learned that, at the factory, when the grey tank was being “plumbed”, workers would take a length of plastic pipe, cut a hole in the appropriate location on the gey tank and seal the pipe in place so that the right amount of pipe was sticking out to be connected to the gate valve. The installer apparently did not care if said pipe was too long as the excess was hidden away INSIDE the grey tank. With the pipe being angled upwards, an overly long pipe could have its’ opening several inches high. This meant that GALLONS of grey water remained inside the tank. Eventually said residual grey water would get “ripe”. LittleHouseCustoms reworked and corrected this problem. The properly fitted drain pipe did a MUCH better job of emptying the Grey water. I understand that many (most?) line workers are paid by the piece so, if they can shave off some time by NOT properly fitting a pipe, it puts more money in their pocket.


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