I find that I get the best mileage when running at 63/64 and 1750RPM. Depending on the terrain that can be 13/14 or 10/11 MPG. From where we live in NE Ohio, when we head south for the winter our worst day mileage wise is the first - the mountains on I77. Conversely, the last day on the way home in the worst.
With our Volvo 420 pulling our 36 foot 5th wheel I get 10-11 MPG, it has a 10 speed and I am happy with that, Class A mileage differ due to size, weight, engine, drive trane, body shape for areodynamics, and rear end gearing, so no set MPG for a Class A diesel or gas.
