I wouldn't spend too much time, worry or money trying to improve the accuracy!! I've owned several RVs & never had one that the those sensors were of any useful information after the first couple times the tanks were filled.
Your kitchen/galley tank will out last the others unless you run excessive water doing dishes.
You'll know the bath tank is full when it backs up into the shower, usually about the time you get all lathered up & is the tank that will fill the quickest. Usually I leave the bath tank valve open when connected to sewer so not dumping every other day.
The toilet has a very distinctive burp/gurgle when that tank is nearing full.
Danny & Linda
Sold '13 Thor Redwood 36 FB
Traded '13 GMC Denali DRW D/A
Currently RV & truckless