My brother just came to visit from Ohio with his new GMC.  It's a 1/2 ton crew cab Denali with the new 3.0 diesel.  I drove it while he was here and it's quiet and has plenty of power.  He's pulling a 25' bumper pull camper and said he averaged 14mpg on the trip out here.  He only had a little over 3000 miles on it, but he said driving it around his local area, he was averaging 28mpg.  I'd take that.  I'm not sure if it was just because it was a top trim level, but man is that thing quiet and rides nice.

Hope this helps, just sharing an experience,

Mike