More than likely, there is still some water at the bottom of the tank that is not usable as the plumbing line that goes to the water pump does not go all the way to the bottom.
Try draining your tank completely with the valve under the trailer. Then meter the next fill up.
Still, if it fills the full 66 gallons, you now know you have available only 45 gallons. So, when camping, adjust to what's available.
2019 Montana High Country 375FL Fifth Wheel
2014 Chevy Duramax HD 6.6 - 3500 Diesel Dully Long bed Crew Cab