There was a thread here that asked what mileages we are getting with our buses. It kinda depends on the motor/rear end combo. We get fantastic mileage with ours (and no one believes me). As for economy... forget it. None are what you would call economic. You are hauling tons of stuff. On the up side, I have heard of few buses that gets mpg as low as comparable sized sticks-n-staples RV's (far down into the single digits). Even our 40 ft BlueBird, loaded to the ceiling with all our stuff got more than double the mileage that my 22 ft Class C RV did on our trip out west. On the other hand, I can get my Class C up to 80 mph but it costs me in fuel.
I would say opt for a diesel engine. Simply because they last longer than a gas engine. then non-electronic diesels are cheaper to work on than the electronic ones... so I've heard.
Whose getting 10+ mpg and what engine is it?
BTW, we like to run primary/secondary roads over interstates. We no longer have to get from point A to point B in a short time so we are slowing down. We are now actually making better time and trips are easier on us. I can't see to drive at night anymore and David's night vision is going as well. Since his heart attack, he is talking taking life easier. I don't know how long that will last but we have decided 150 to 200 miles per day is our planned travel days.