My F250 with an FE 360/4BBL/4speed manual with 3.73 gears gets 12 on a good day empty with a tailwind. Hauling, 7.5 is pretty typical when I'm towing on the highway, but then it's typically a 9k-12k trailer with a lifted 4x4 on it and doing 65-70. That's what I built the truck and trailer for, highway towing. I really want to put a Cummins 6BT in it with a 5speed though, it would be good for 17-18mpg empty and about 14 towing if my friends various trucks are any indication.
Back before the 24v Cummins came out, there was a company doing a Ram 3500 4x4-based camper, huge thing with a Cummins and 5speed, that actually was tested by the magazine reviewing it to get 14mpg on the highway. Unreal, I got 6mpg out of a mini-home with a Ford 400.