Problem right now is that the tank is about 70% full and if I manage to dislodge whatever is causing the problems from the outlet end of things, there is gonna be a whole mess of stink in the neighborhood. I intend to inspect the valve and find out the exact problem after getting it drained, which I hope to acheive with a plumbing auger. When the valve first got stuck open like that the tank drained completely, so it was not plugged (like a cutout falling into the wrong place would do) then. :(
The RV is not new, it is a pre-owned 2003.
I turned off the water pump and held the toilet valve open; all I see is an elbow joint directly below the toilet, no water level.