Well, to be honest you only have to use the camper once with poor black tank habits (insufficient water is the biggie) to totally screw your indicator probes.
I was referring to using a septic type cleaner (instead of a holding tank de-stinker). Septic type cleaners (Rv-Digest-It by Unique or RID-X) require a full tank and a minimum of 72 hours to work (cook). I use a week when winterizing (both tanks and 1/2 quart of RV-Digest-It in each).
They work by enzyme and a specific type of bacteria action to break down solids and caked on (well) crap.
Prevention is always better. ONCE you have clean tanks, 1 cap full of Calgon Water SOFTENER and a cap full of laundry detergent (and a few gallons of water) after EVERY dump/flush.
A more permanent fix is to replace your OEM probes with Horst Probes.
I now recommend using black tank style probes in the Gray tank too as I got hair fouling in my gray tank using the spiral.
Horst tank Probe Installation
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