CA Traveler wrote:
Black tank has a large roof vent so not likely to be the problem. Tilt the rig and drain the tank, then you can work on it including a snake. Good time to also replace the valve.
Not sure what you consider as "large" but the black holding tank will have a pipe diameter that is half of the tank outlet. Outlet on black is 3" so the vent size will be 1 1/2".
To make this a bit confusing but easier for manufacturing purposes the grey tank will use the same 1 1/2" pipe for roof vent even though the grey tank has 1 1/2" outlet..
To help make things a bit easier to locate the tank vents the OP is looking for a small round "mushroom" looking cap on the roof..
Many variations of design but here is what many plastic ones look like..
To snake the vent, one will have to remove the entire cap assembly..
If vent is clogged, OP may have to open the toilet valve to relieve the vacuum as the tank is being dumped.
It is also possible that the vent pipe may have slipped down into the tank just enough that 3/4 full it no longer vents..
