Cruise control costs 1 MPG in my experience. Using your foot, you can do a much better job of anticipation and coasting.
The biggest problem is aerodynamics, but mostly we are stuck with what we have. Driving behind a big truck helps quite a bit but there are obvious issues there.
There is a lot of DIY info in the forums at ecomodder.com. Here is a pretty radical aero treatment on a Class C:
http://ecomodder.com/forum/showthread.php/aero-rv-custom-boat-tail-95-ford-e-18151.htmlYou might pick up a little with an air dam under the bumper, rear fender skirts, and other little mods. A manual transmission would probably help, but maybe you can get something that lets you control your auto trans.
