Running mostly 70 MPH, I averaged 12.7 MPG, with the trailer. Not terrible, but not winning any awards, either. I ran out and back, so any hills or things should be accounted for. 709 miles, round trip. This trip was small hills, with brief stretches of flat. I opted for the 5.3L truck.
Bad me, but I used cruise control while towing. Some hills, the speed fell off a few MPH, but I could put my foot on the pedal to bring the speed back up. Very annoying that cruise does not hold its set speed.
The transmission did use engine braking when slowing, because I had the tow mode engaged.
The trailer itself did settle the rear end of the truck a bit.
The most annoying part of towing with this truck is the bad gearing between 1 & 2. The truck gets moving, then there is an apparent dead spot between moving and going.