Just returned from 9228 mile trip. West coast to East coast and back with some trips up into Canada (even made it to Newfoundland - great mpg on the ferry ride).
We have what I feel is a pretty aerodynamic Aspect 26A B+, v10 - E450, which is 27ft (spare tire to front bumper) long, (correction): 12450lbs ready to travel, now has 30,840 miles on the odometer, 70lb air in rear tires and 60lb air in front. Pulled a small trailer with a 465lb scooter in it. We travel with full fresh water tank because we boondock and we empty the grey and black water tanks when they are between 1/2 to 2/3 full. Using mobile 1 5w/20 synthetic motor oil. Had front end alignment and the brakes serviced and new fluid just to be sure there was wasn't any drag from sticky calipers.
I drive 60 mph (sometimes higher on two lane roads with traffic) and kick it out of cruise up the hills to keep it in overdrive a long as possible and let the speed drop as we climb. Use tow/haul sparingly. Scan gauge helps to drive more efficiently. Think that the bugs probably account for some of the drag...some of those bugs really splat and stick.
WE AVERAGED 8.9 MPG. (Think that is the best we can get with this rig).
Gas was the most expensive part of the trip but I wouldn't travel any other way. Loved it. Like to drive and see the country. Great sunsets and sunrises and met some wonderful people. Love making these types of memories.
Always looking for ways to improve mileage but short of trading for something smaller and lighter don't think I will find any. It is what it is...;-).