A very common problem with all RVs. Even if you don't think they are dirty they still can be. The slightest bit of residue along the side of the tank can provide a current path and cause the monitor to read incorrectly.
There are a few solutions--some more expensive than others.
The Horst Miracle Probes aren't that expensive and provide some relief: Horst Mircacle Probes - Get accurate reading on your RV's holding tanks
I've been working on my own capacitive sensor:
Unfortunately I'm stuck at the point where I need to test the sensor at a campground and I've been so busy "camping" that I haven't had the chance to do the testing ! LOL.