I have an '03 2500HD SB/CC with the 6.0L/auto/4.10 gear setup. I think it does just fine towing my 8,500 lb. Blazer and trailer combo and has never had a problem maintaining (or exceeding) the speed limit.
Now granted, we don't have any moutains in Ohio but the Eastern and Southern parts of the state get pretty hilly. On I-70 heading into a 10 mph head wind I had no problem maintaining 75 mph with the cruise control on with the trans shifting out of OD and into 3rd on most hills (no big deal). This was with two guys (about 450 lbs. total), firewood, food, and camping gear for a long weekend, and the Blazer on the trailer. For comparison, my buddy was driving his Wrangler with a swapped in and modified 4.0L HO, 4.10 gears w/ 33" tires, and he had to downshift from 5th to 4th to maintain speeds on the same grade with just him, his wife, and normal trail gear in it.