I agree with Passin. I just don't understand the issue.
When I look at fuel costs/miles traveled as relates to total RV costs including maintenance, it's a small part of the budget.
Cost of fuel you have very little control over, other than trying to save 5 or 10 cents a gallon, which is about 2-3%, based on $3.50/gal.
As everyone has said, slowing down will save you fuel. I usually don't. I'm usually on a schedule, and want to get there, so I run with traffic, usually 65mph.
When I figure my insurance costs, site fee, food, etc...saving 3-4gals of fuel becomes a very minor expense...one that I just don't fret over.
Jim