[NH_Bulldog]

I wouldn’t trust the tank gauges. On mine, the tank is only as deep as the frame rails - 8” or so. Like JRTJH said, just a little off level and gauge reads wacky.

Consider these two experiences from a recent long trip with no water connection:
My parent’s motor coach has a visible tank in the side compartment. The 1/3 sensor is halfway up the tank, the 2/3 sensor is 3/4 up and the full sensor about an inch from the top. This means that the tank shows empty until it reaches the half full point but the gauge shows only 1/3 full. When it reads full, there is still room for more.

On My 43 gallon tank the gauge read 1/3 full, I filled 4 water cans (6 gallons each) and could only get 2 1/2 cans in.

My routine on trips now is that I drain the tank before we leave, then add 6 gallons for road use. If we are boondocking, we typically plan on 6-8 gallons per day and fill accordingly.