I suppose you have the conductive type level sensor, but in case you don't:
I have the One Place central panel which uses LevelGuard external sensors two-face taped onto the tank. Mine started to report 1/3 full (when empty) out of the blue. Turned out that an unrelated chassis wire harness had drooped over time to the point that it was now close enough to the top sensor in my black tank to "trip" that sensor. Any one sensor tripped is considered 1/3 or 1/4 full - depending on the total number of sensors. A wire tie took care of it.