Towed my 32 foot travel trailer with my Chevy 2500HD 6.0, 4.10 gears, 4L80E this weekend.
Averaged 8 to 9 mpg. 120 miles each way plus a few 10 mile trips without the trailer. I ,most likely, averaged about 40 to 45 mph.
Total GCVW was about 14,000lbs, 2000lbs under the truck's limit.
Could care less about gas mileage with this setup. Spent the weekend on the coast of Washington in awesome weather and the smiles on my kids faces at the campground is priceless.