TAB 320s 2021 Gray water tank indicator not working?

We love this camper. On a recent trip the gray water tank indicator started showing 2/3 full all the time. Thinking the sensor might be gunked up, I poured some Thetford Aqua-Kem holding tank deodorant in, which is also supposed to help dissolve any stuff that might be in there. Left that in for 4 days. Then emptied it, and it still showed 2/3. Filled the tank with clean water until it backed up out the shower drain...it showed full. Yay-that's correct! Emptied it and it shows 1/3. Repeated the filling/emptying and still shows 1/3. Any ideas on what to try next? THANKS!