Originally Posted by sjk1998
After the sink or tub backs up with grey water does it eventually drain or stay backed up as if your grey tank is full?
Maybe go on the roof and take the cap off of the vent and run a plumbers snake down inside to ensure that there is no blockage of any sort.
Using a plumbers snake on any plumbing in a rv wouldn't be recommended, lots of plastic.
A water hose with a pressure nozzle would be safer than the snake.
If insects, birds or just debris has collected around those cheap vent caps it won't be vented & not drain.
Fortunately it's the grey tank, still yucky but not poop, so if needed you could open the drain & shove a hose into it backwards & possibly move any obstruction.
It's also not uncommon for the factory guys to drop the cutout from the tank into the tank & finally wash/float to the drain keeping it from draining.