I agree with Porky. I usually tow a Suburban and drive 1-2MPH over the speed limit on the interstate using the cruise. I also always travel with a full H20 tank. 7 to 8 MPG.
I have tandem axle's and have the Eaton 10 speed auto tranny. Head winds seem to make the biggest difference in MPG. I can drive over the Rocky Mts and still get the same average.
If I slow down to 58 MPH, like on a state route road, I'll get around 9 MPG. We are pushing a big square box down the road, not the greatest aerodynamics!
2007 United Specialties 45' garage coach tandem axle DD515 w/Eaton 10 speed auto