We have a 7.4 in our 24 foot C with no tow. We have gotten 10 mpg avg. for as long as we have had it. However We had an Airstream trailer we pulled with an old Chevy 6.2 diesel for about a hundred thousand miles. It had little power so we always just ran with the trucks. Seldom in the fast lane. Kind of stick with my old driving habits and drive the C the same way. I think driving habits are the difference in fuel mileage for most of us. If we have to make a fast trip and get out in the fast lane at 65-70 mph we drop to 8.5. Add a tow and it would drop another mpg.
We spent most of our money traveling... Just wasted the rest..
On the freeway, go as slow as you can without dangerously impeding traffic. Don't try to maintain speed up hills, let it slow a bit going the small hills.
Off the freeway, go light on the accelerator - no rabbit starts. Try and maintain a constant speed where you can. 40 MPH should be close to your most efficient speed - choose routes where you can do 40 MPH with minimal stop lights. Look ahead and manage your speed to avoid stopping at lights when you can.
Keep tires aired up at proper tire pressure for the weight they are bearing. Don't stack up extra items on the roof (extra wind resistance). Install Airtabs (might help, won't hurt, but probably too small a gain to be measurable).
Managing how much accelerator you use has the largest effect on gas mileage by far.
Gas mileage has so much to do with the way you drive, I see so many people that drive up behind other cars and then hit the brake, Try to keep a steady speed, And when your coming up to a red light try and lay back a little to keep your speed up as much as possible instead of running up full speed to the red light and stopping just to have to pull back out and build your speed back up, Good gas mileage is in the driver not so much the vehicle.