For what it's worth, I have a Tiger CX (Class C Extreme) Motorhome built on a 2008 F-350 4x4 Extended cab w/ a 6.4 L Powerstroke with about 3300 miles on it. My gross is 10,280 lbs. and I am getting (according to the built-in computer) around 12.8 mpg. I usually run around 70 on the highway. I...