The higher the speed the greater the resistance from the mass of air that the RV has to push trough and aside, the more fuel it is going to need to counter act the resistance, then you have the round corners and a flat rear cap that create an incredible suction on the back of the RV, and that adds to the fuel consumption.
Find a unit that suits your needs and make a budget for vacation fuel, plan the outings in such a manner that you account for the terrain that you will be traversing, flat small hills say 7 mpg, mountains 4 mpg and you will have a guesstimate of the fuel needed, and just relax and enjoy the view.
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