We have used the black tank just about exclusively for potty (instead of campground toilets) on motorhome trips for the last 9 years - including right now in the middle of our 3rd over 7000 mile long trip.
Here's what we do:
1) Never hook it up while using it - even in campsites with full hookups. We let it get full and empty it only occasionally whenever it's full every few days.
2) We always use only one-ply septic safe toilet paper in it - never two-ply septic safe toilet paper or three-ply septic safe toilet paper.
3) We always leave little bit of standing water in the toilet (about 1/2 inch) when it's valve is closed - even when the RV is in storage.
4) We always use Happy Camper powder additive in the black tank when preparing it for use after each dump.
We get steady-eddy performance out of our black tank year after year doing the above in all kinds of camping temperatures. Even the gage is very accurate doing as we do - even though gage accuracy is not real important with our RV because we can look directly down into the tank using a flashlight shining through the toilet hole.