I just returned from an atv riding trip in Logan WV {Hatfield McCoy trails}. I rode with a friend who has basicaly the same truck as me, other than the fact that his truck has the 8100 gas engine with an Allison and 4:10s, 2wd, mine is a DMax Allison, 3:73, 4WD. Both trucks have 265's, are reg. cab long beds, and relatively close miles, mine 97k, his high 80's, his is a 2002 2500HD, mine is a 2003 2500HD.
We were towing a 22ft tailgator camper/toy hauler with 2 4wd utility quads inside, and gear and food for four adults, the water and waste tanks were empty, we estimate the weight of the camper to be around 6500lbs as towed. We were also using a cab top air deflector. The best hand calculated MPG we got was 7.1mpg, the rest of the time we were between 6.4mpg and 6.8mpg. It was 392 miles each way, and it took almost 3 tanks of fuel to get there and about 2 and a half to get home[ had a little left]. The Rig also spent lots of time in second gear at 4000+ rpm's climbing the hills on the freeway, it did however maintain 65mph the entire time, regardless of the terrain.
It would have been cheaper to drive my Duramax down there by myself {towing the same load], rather than splitting fuel with my 8100 buddy. I also doubt that it would have come out of overdrive on any highway grade that we encountered, [it is chipped though] based on past mountain towing experience in it. I sure am glad I spent the extra money 6 years ago for a diesel. The 8100 did the job, but I prefer the diesel.
I always tow in overdrive, and use Tow/Haul when towing and NEVER exceed 60hp setting on the chip when towing. Rarely have I seen trans temps in excess of 200 degrees, only a couple times when towing in heavy stop and go highway traffic. My buddy with the 8100 towed all the way down and all the way back in tow haul overdrive with no issues. That being said, If I did not have a Allison, Torqueshift, or Aisen, I would play it safe and keep it out of overdrive when towing, as none of the older auto OD's [E4od, 4l80 etc.] were realy made to be towing in OD.