I get 14-15 mixed city/highway. Highway, it's hard to say since i havent run a tank of pure highway. On my one long trip in summer 2010 was bumping on 17... but i had my motorcycle in the back, and i got healthy ticket in Canada.
Fyi, Ihave a class 3 hitch on the truck and it's wired for a brake controller.
Really, the only reason i am thinking of selling is that I drove a 2011 Yukon xl with the 6 speed hauling a 6200# trailer a few months back... i fell in love with the engine trans combo. I started dreaming of an 09+ crew cab silvy or sierra on the way home.