All of the analysis is futile. You cannot control the wind or terrain although you can control speed. In the end the cost difference will be negligible over the course of extended driving.
Put the calculator down and enjoy your drive without worrying about MPG's.
RV's are inefficient and use a lot of fuel your mileage will be bad. This applies to all types and brands. You can't fight the aerodynamics of moving a big box down the road
If you want to conserve drive slower, even then your actual cash savings will be minimal.
At some point you have to pay the fuel bill to play the RV game. Do it with a smile and enjoy the adventure.