As long as you use the actual gallons pumped, it doesn't matter whether or not you filled the tank completely.
In spite of being as accurate as you can, the mileage really doesn't matter because it will vary from day to day because of weather, topography, traffic, and the type of fuel and the percentage of ethanol, which varies from tank to tank with a maximum of 10%, but could be as low as 1%. Basically it is best to just accept the fact that with ANY Class A RV, the mileage will be poor based on ANY standard of measurement.