[Carl n Susan]

The shower drain is the low point of the circuit. The sink and washer are all higher so water into them, when the tank is full, will go down. It just raises the level in the shower.

I think you have a full tank. You can bail the shower and then run some water in the sink and see if it goes back up. Otherwise close all the other drains connected to the sewer hose, undo the sewer hose and try running a snake or something up into the tank to see how far it goes. Maybe then you can gauge the open/close status. I don't know if this is practical or not.

If it were me, I would drop the back 3 feet of the coroplast in the dropped frame section and look at the valve. Not too tough to do and likely quicker than snaking the line.