Like almost everyone else, I too have sensor inaccuracy.
I try to follow this routine.
On the Clipper, we dump waste and fill fresh water at the same time/stop if possible.
I can then monitor the black tank with the flashlight down the hole, and the freshwater tank also with the flashlight and a mirror on a long handle. The fresh water tank is in the basement, up between the raised frame rails and with the mirror and light, the water level is easy to determine.
The Clipper is impossible to overload, so we run with 105 US gal.
water full when leaving any campsite or our home base.
If the fresh water tank is getting low, we know the gray tank is getting full
and its time to dump.
The gray water tank seems to be limiting factor on most RV's.
We have never been close on the black tank as it seems that most of the water goes to the gray. (depends on the tank capacities
I have never seen where the sensor is on the grey tank on my unit. But if I ever have reason to access that area, I would like to update it to the Horst mentioned in the above post.