Well, my 2016 6.2 gas got 8.6 mpg on a trip to Ash Falls, Nebraska, just this past weekend.
That's combines, 500 miles out thru 20 mph headwinds and 500 miles back being pushed by the headwinds.
While fighting headwinds it was more like 7.5 mpg. With the wind at my back it was more like 10.3.
No wind, carrying the Lance 820 which fully loaded is like 3000 lbs, usually about 9.6 mpg.
Hope that helps. I got this F250 specifically with the truck camper package to carry a camper. Handles it just fine. I'm over GVWR, but well below tire ratings.
2017 will have probably 150 lbs more of payload for the F250. F350 is not necessary, in my opinion. Some states and/or cities will charge more to register a 1 ton truck.
My truck is registered for 12k lbs, but still counts as a 3/4 ton for registration purposes, which is the only thing the law cares about.