A number of years ago I bought a toy hauler that was supposed to have a 75 gallon water tank, not including the water heater. The first couple of times I used it, it never seemed like I had any where near that much water available.
When I got underneath the trailer and looked at the tank I found that the vent hose was located on the side of the tank, about an inch below the top of the tank, and the pump pickup line was also located on the side of the tank, about an inch above the bottom of the tank. Using the area of the bottom (and top) of the tank, I calculated that one inch of depth in the tank would hold about 10 gallons of water. Turned out that when the tank was "full", at least 10 gallons of tank capacity went out the vent hose, and the last 10 gallons in the tank couldn't be pumped out, so I had only 55 gallons of usable water from a 75 gallon tank.
I did have the dealer move the pump pickup fitting to the bottom of the tank, so I was able to get the last 10 gallons from the tank. Couldn't do anything about the upper 10 gallons without dropping the tank.