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
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.