10 Mpg seems a little on the low side but it all depends on your speed, the terrain, gearing, and your MPG when empty. My 83 F150 with 302, AOD, 3.55 gears would get 16 Mpg empty at 70 Mph and about 10.5 to 11 at 60 Mph with a 4500 lb Prowler TT.
My 95 with the 300, 5 speed, and 3.08 gears gets 17 Mpg at 70 Mph and will get 13 Mpg with a 4500 lb Salem fifth wheeler at 60 Mph. The Salem and my truck are pretty aerodynamic. BTW I didn't use OD in either rig when hauling.
Hope you had a great time. I had a great summer camping and just winterized the rig until next year.