That's a bad one. Forget the gauge indication. Can you see down the toilet (with the water shut off) to actually see the level?
There is a product made by Valterra called RV Hydroflush which is an adapter that connects to the sewer outlet. (Camping World has it. I like the 45 degree model) It has a hose connection that allows one to send water BACK into the Black Tank to break up anything that might be clogging the outlet and help in the flushing procedure when dumping normally. A sewer hose is connected to the device and it's clear plastic so you can see when the outflow is clear.