My superduty makes 16 mpg with camper shell with CC set at 70mph and 10 mpg with about 5000 lb loaded camper going to Sierra usually with moderate traffic.
My camper stands 12' tall and traffic is partly to blame for lower mpg.
I did some heavy hauling and observing fuel consumption, it is frontal area that makes biggest impact on mpg, while the weight makes difference in city traffic, while has almost no impact in freeway driving.
When I tow my dovetail trailer, the 4500lb makes marginal impact on mpg. I believe dovetail improves aerodynamics of the set comparing to pickup alone.