Before getting our Surveyor last year, we've pulled a 4500lb boat (and tents) to all our outings. We started with a 96' 1500 Surburban, went to a 01' Denali, then an 07' Escalade (same drive-train as the 07+ Denali).
The 07' Denali/Esc 6.2 w/6spd is definately the best setup we've owned. At 53k in our Escalade has held up great. 50% of the mileage is towing here in the Tahoe area, and it is always a 6000 ft elevation change to get to where we previously boated and now camp. The worst problems we've had were all software glitches (2 of them over 4 yrs), and a recent recall for the heated windshield washer system.
The surprise we had moving to a TT was the drag created by the profile. Mileage suffers not due to weight (our little TT is about 5k loaded), but wind resistance.
Although a diesel is the best for towing, the GM 6.2-6spd combo is a decent gas setup. The 6spd makes a big difference on hills because in the previous 4 spd vehicles we had, going up hills, the engine was either bogged, or screaming.
Dave & Tami