Often the tank’s water outlet is on the side, near, but not at the bottom. For a given size tank, you might be able to put that much water in, but you can’t get that much out, except maybe if you park the motorhome on a hill, with the outlet fitting on the downhill side. Sometimes maybe something like 5 gallons could be trapped in the tank since the water outlet may be on the side, not the bottom. It’s a joke the way they build these things.
Does your TS have a tankless water heater?
2020 Sunseeker 2440DS on 2019 Ford E-450, Trekker cap, Topaz paint