I'd suspect the sensors or the wiring to the sensors. In the freshwater tank, there's not much that will cause sensor problems, so I'd suspect wiring breaks or faulty connections. In the grey and black water tanks, accumulation of "sludge" can fool the sensors into seeing more in the tanks than is actually there, which would give you a more full reading than you actually have. If, however, you are seeing "empty" readings when the tanks are full, then I think the wiring is the problem again.
It's fairly easy to get to the wiring for the grey and black tanks in my trailer. Perhaps you will find that to be true in yours as well. Look under the trailer and along the sides of each of the tanks (mine are on the front sides). You should see wires connecting to what look like bolts (those are the sensors) and those connections can become corroded. You could disconnect, clean those, reconnect and see if your problem goes away.
I cannot get to the wiring for the fresh water tank in my trailer without removing the tank. Maybe you would have better luck.
Hope this helps.
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