Everything said here about the 10cyl gas motors is true - they will pull stumps but nobody I know with one ever got better that about 9mpg average.
And most people posting here are getting around 8-10, I assume most are on gas also.
I must be the real excpetion here:
2006 Dodge 2500 with 3.73 rear gear with common rail 5.9 cummins diesel. Curently pulling 7400lb empty (8500lb loaded) trailer on flat ground (everything is flat in Louisiana
Keeping it under 65 (no cruse - although as flat as it is here it may not matter). I am getting 13mpg (sometimes on back roads at 55mph I am getting 14mpg) the truck usually get over 20mpg empty in same driving conditions.
I have to admit after owning a few campers my last one was a 7600lb V-Nose bumperpull and it too got around 13mpg (big improvment over previous rig "Brick trailer - flat front" with Ford Powerstroke trk) I swore I would never own another flat front brick trailer - it seems that around 60-65mph I would hit a wind resistance wall and I had to push the petal down quite a bit and sucking a lot more fuel just to go a few miles per hour faster with that brick.
BTW with the 6500lb Brick and a Ford 6.0 Powerstroke - my previous rig, I could only average 10mpg on flat ground. I think the cummins may have done better even towing the brick.
Anyway so I figured after learing these lessons my future camping rigs - stay under 8K pounds, stay under 65mph, and buy arodynamic trailer.
My current 5th wheel not much taller than truck and pretty arodynamic front cap horse trailer style (getting 13mpg):