It gets approx. 12-13 hauling our camper (3,500-4,000 lbs. loaded).
I get 13-14 unloaded and 10-11 loaded with the same weight you mentioned with a 460 Ford..Now if I could put that 6-speed or 10-speed transmission behind the old 460,it would work far less..
I won't argue a diesel has more power and better gas mileage but I will note that diesel engines is not needed with a meager truck camper and a modern gas engine with a modern transmission..My 460 does a great jog with 36-4000 pounds in the back,lots of compression for down hill and plenty of power for up hill...
With the speed limits as they are,a diesel has no edge in power with a modern gas engine..They both will go over the top of Idaho mountains at the same speed carrying a modest truck camper..Now throw in a big 5er and the edge definitely goes do the diesel with all the weight..