Thanks for the links to various RV discussions and data slowmover. Doing a quick and dirty tally, out of all the class C results I read from all those sources, the average mpg for this particular sample of class C RV's, is about 8.5 mpg so I guess I should appreciate the results from my aeromods more.
The Sprinter/Mercedes vans are reporting 16 to 18 on average and they are mostly shorter and lighter with engines less than half my displacement and horsepower. That's pretty good.
One guy with a 27 footer and a Cummins 6 banger says he gets 15 to 17mpg, which is the highest I've seen so far, which reinforces my own opinion of Cummins drive trains. They may not be as strong under a big load as this 7.3, but they are killer dependable and generally get great mileage. As soon as it cools off a little I'll continue my efforts. 20 mpg straight and level is my goal. Aim High and reduce the Air Force.
We'll see.