He can take the control off the wall then try to figure out which wire is which coming out of the control. A wiring diagram of the control might help there too.
From doing that, he can figure out which wire is ground, and just run a jumper wire from a ground point inside the coach somewhere and see if the tank levels start reading.
If that control performs other functions, and those functions work, it may be possible your tank sensors are blinded. Do a YouTube search on “The Geo method” and/or “poo- pyramid” - there is a guy that has tap two part series of videos explaining in layman’s terms how to clean your tanks, get your tank sensors working, and save you bunches of money - not to forget that your waste tanks and bathroom won’t stink any more.