It sounds like the only things that are currently wrong are some sensors that are not accurate and a stuck valve. From your post it sounds like the tank is empty.
I would look under the fifth wheel and see if you can see the tank and the valve. Now find a way to tilt the trailer toward the valve to completely drain the tank.
If the drain comes out of the back of the tank, jack up the trailer so that more stuff drains out of the open valve. If it comes out of a side of the tank, find a way to lift the other side.
Once the draining stops tilt the trailer in the opposite direction so the contents in the tank run away from the valve. Now while everything is draining away from the valve, go get some tools. If I remember correctly you will need a 3/8 inch wrench and a 3/8 inch socket and ratchet.
Unscrew the four bolts and unhook whatever is holding the exit pipe and stinky slinky onto the valve. Now pull the valve out and take a look. If you did this right you will only get a dribble out of the tank as you have the outlet tilted up so the effluent is drained away from the valve. Find the problem and fix or replace the valve. They only cost about $14.00 so they're not too expensive.
You really don't want to let this go or you most definitely will wind up with the dreaded poop pyramid. The solids will drop into the tank and stay under the toilet while the little bit of water from the bowl will drain out.
Good luck with it and let us know how you make out.