Static campers have the worst problems with sensors because without the agitation of movement, keeping the walls and floor of the tank clean is a challenge.
You must install turboflushers (ESPECIALLY) if you use a pump out service. The service only sucks out the liquid from the tank and can easily leave solids behind. Eventually you will need a clean-out service (NOT a pump out service - very expensive) or a tank replacement (even more expensive).
So, you need to treat your holding tanks like a septic tank more than a holding tank. Septic tanks use bacteria and enzymes to "digest" the solids in your tank and turn it into a liquid that will flush out.
Use a bacterial treatment like RV-Digest-It by Unique RV Digest-It Holding Tank Treatment
or Rid-X RID-X® Tank Septic System Treatment Products for Septic Maintenance - Official Site
to maintain your tanks. You will need to allow the stuff to work for 72 hours to eat the scum off your walls (causing your bad readings - NOT TP!) and to soften your "Tower of Doom" under the toilet drop pipe.
Worst case you will need to replace the OEM "button" probes with Horst probes to eliminate false readings.
Horst Mircacle Probes - Get accurate reading on your RV's holding tanks
My install here: Horst tank Probe Installation
If you go this route, I suggest BLACK tank probes in BOTH the black and gray tanks. I had issues with long hair from the shower fouling the spiral coil on the gray tank probes.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW