I've given up trying to answer that question: "What kind of mileage do you get?"
"It depends" is the answer.
With camper on, no apparent wind, moderately level terrain:
65 mph: 13.5 mpg
70 mph: 10.5 mpg
No camper:
Non-freeway streets: 15 mpg (in southern Arizona)
Non-freeway streets: 11.6 to 12 (in south Puget Sound)
(There is a huge difference in topography and speed limits.)
Recent 1,400 south-to-north mile trip with camper on and towing a small enclosed trailer: 11.3 mpg
This includes lots of desert, mountain ranges, headwinds, tailwinds etc.
Truck & camper: 15K pounds.
4-wheel drive. 3.73 axle CC diesel dually long bed.