This tank cleaning service works. It restored my stuck sensors to working condition and did a great cleaning, even though I thought they were pretty clean before. Used him several times and will use him again. kleentank.com
He attends various rallys - check his website. I have no affiliation, other than a customer.
2019 Windsport 29M
No matter how many times we flush black tank and drain thoroughly monitor shows 33% full.
What is wrong?
You might try checking underneath your rv. If you know where your black tank is, you may find two sets of sensors are on the tank. One set is wired, the other is not. The wiring is fastened with screws which easily are removed. Just swap sensors moving the wires to the corresponding sensor (top to bottom). I think they built the tanks with the back up set for this purpose. At least that is the case with my Quantum. Otherwise you probably have TP stuck to the lower sensor giving the false reading. A good high pressure wash down your toilet might fix it.