[DK on the road]

I'll address the valves opening or not. There is a manual allen wrench to open and close the valves to dump on each valve. I have damaged both of my dump valves by holding the safety and dump button too long. Curb side and Drivers side dump valves. It may have stripped the worm drive, but non the less, they didn't work and I had to resort to the allen wrench. They are not hard to change out, just make sure the tank is completely drained and I also tipped the coach away (towards the Drivers side) to make sure no effluent came out while I worked on it.

These 2 items should be different and not connected, the best I can tell

As far as sensors, there is another thread here on this site that talks about a universal monitor that other have used, if the problem is with the sensor itself.

I'm not an electrician ( very armature) but I'm confused that your getting a full reading. Others will correct me, but that either means your getting power at the monitor when you shouldn't or your not getting power when you should. My black tank was reading correctly and now shows empty all the time.  I'll watch for information from the wiser Guru's here on what we should look into.