Unfortunately, the tank monitors area not very accurate, just by design. If you look under the trailer at the tanks (if they are not enclosed), you can see the probes on the sides if each tank. It seems as though the installers are not very precise in their spacing. Also, a bit if corrosion, some stuck tp, or other foreign matter can cause a false reading.
You should be able to start to have a good idea on water usage through trial, and error.
We have a 2012 28BH and notice the probes arrear not correct, even from the second or third trip.
Joel and Teresa
2016 Sabre 34TBOK
2006 GMC 3500 CC LB DRW 4X4