If your tank has been reading incorrectly since the first time you used it, there's a possibility the sensor wires may be connected wrong. It you can see them from underneath, from the top down, they should be red, green, then yellow and white near the bottom of the tank. I just found two wires switched on our gray tank sensors. They were connected wrong from the factory, so it showed full, when actually it was only 2/3rds full.
As above, rinsing well with a wand down the toilet is the easiest way to keep them clean. Our's still work correctly after 8 years.
Every day's a learning experience, though sometimes not what we expected.
2012 Jay Flight 19RD
2016 Ford F150 XLT 2X4 SC 3.5L Eco Max Tow
2010 Tundra TRD DBL Cab (Traded)
1 Spoiled Dog