Asking about fuel milage is a bit like asking how big the fish someone caught is. The truth is it depends on so many factors that the numbers can be all over the place. How fast are you driving, plains or mountains, wind, etc.
As a data point, on reasonable flat ground driving on the freeway at 68 mph with no wind I will average 9.0 mpg with my camper. Give me a strong headwind - 6.8 mpg. Give me a strong tailwind I can get up to 12.0. Drop the speed, milage goes up pretty significantly, raise the speed, it goes down fast
2013 F350 CC DRW, 6.2, DRW 4.3 axel. AF1150 camper. Entire rig loaded up is just under 14,000 pounds