I finally took my camper on a trip that was long enough to check the gas mileage. I won't check it again real soon. I averaged 6 MPG towing about 6200#. Granted, I looked good since we have a Screw and not an Avalanche. Has any one else gotten worse mileage? I was doing about 60-65MPH. I think a superchip might be helpfull.

 

Sounds like 285/75R16's with 3.55 rear end is your problem. Your towing capacity went way down with the larger diameter tires. If you were not all ready, try turning overdrive off when towing. You can band aid the problem with engine or tranny mods, OR...... regear the rear end, OR...get smaller diameter tires. Ya can't rewrite the laws of physics.

 

I have almost the identical truck, only with the parchment interior and I towed a 9,200# (27') camper to South Padre pulling 60-65 and averaged 10-10.5mpg. I did have to upgrade to 89 octane because of the high ambient tempertures making the truck want to ping on grades. I will say this, the truck was maxxed out and at times in cross winds would get a little squirmey because of the overloaded condition. But mileage wise I was happy because my 96 F250 4x4 with 460 E4OD got the same mileage as the Screw and it was nowhere near overloaded. Empty I get a consistent 15-17 mpg using 87 ocatane and that is at 57,800 miles. I only wish they would offer a 3.73 LS ratio like GM does, I think it would really give the truck a good boost.