I have a 2006 GMC 2500HD Sierra Crewcab 4 wheel drive with the Duramax and Allison and also have a 2006 325BHS 5th Wheel. The truck pulls the trailer like a charm, this trailer weighs about 11,500 lbs loaded. So the truck is not over the front and rear GAWR but a bit over the GVWR, but under the max towing capacity of the truck. It could use some air bags as it hangs a little low at the back.
Saskatchewan is quite flat but we do have some valleys and truck doesn't miss a beat pulling through them.
I have put a 4" exhaust and removed the cat, and have a Banks 6 Pack with the Banks air intake.