I recently purchased a 5th wheel. I can easily emty the grey water tank but the black water tanks stays 2/3 full even after I add chemical, a good amount of water and then pull the valve to the black water tank.
Just a thought, but have you actually visually checked the tank level?
My guess is that your tank empties, but your sensor may be
A bad sensor reading can drive a person crazy. If I were a gambling man, I'd say that was your problem. But...? I've been wrong before. When was that? July of nineteen seventy....something....
What I think maybe has happened is that somebody was parked for a long period of time and left the valve to the black water tank open and that there was not enough water in the tank to flush all the poo poo out.
If that happened it is going to be difficult to clean out. You probably need to buy a wand at an RV store that is specifically made to flush black water tanks out. It will attach to the end of your water hose and you stick down into the black water tank and proceed to flush it throughly. Hopefully you will be able to get it all out. Make sure you use an old water hose and not your good fresh water hose.
The best way to keep this situation from happening is to always close the vavle to the black water tank while camping and empty it when required. Also follow up at the end of your stay with the little wand on the end of your hose. I also rinse out my sewer hose by letting my grey water tank fill up and dump it right after dumping the black water tank. Lots and lots of luck with the black water tank if it's full of *$%&.
datsunsott, I forgot to mention that once you get your black water tank working properly, you need to make sure it has plenty of water in it when you dump. Water/liquid is the key to flushing the holding tank out.
I had a similar problem wiith my first RV..class A winnebego..the WAND was magic. You will find that the name of what builds up in the tank is called "Tank Cake". The use of wand and keeping water in the tank will hwlp. The more you use the system the better it works so get the heck out of the city and camp it up....Doc.