Chevy Offers (or did offer) a 3/4 ton Avalanche which has a decent towing capacity, between 10,000 and 12,000 lbs. Unfortunately, they are stuck with a thirsty8.1L gas engine, and haven't been released in '07 model year, so this might be the last chance at getting one.
The '07 models are currently only available as 1/2 tons and are limited to, at best, 8200lbs. Teeny trailer, that will be.
I do not believe that Avalanches are suitable for towing 5th wheels because of the way the weird "roll cage" overhang thingy will get in the way
There are plenty of non-Avalanche crew-cab pickups that are more capable. If they insist on an Avalanche and intend to tow with it, try to talk them into getting a good deal on an outgoing '06 2500 model. Dealers should be discounting them about now.