Issues with GMC 6.0 L

I have a 2011 Chevy 3500 SRW 6.0. Over the past two summers I have put over 13,000 miles on it in the Rocky Mtns, pulling a loaded (12,000lb) fiver. I now joke about the little engine that could, because that truck/engine kicked butt. I averaged about 8-9mpg in the mtns, not bad considering I get 11-12 just driving around the city (in flat Michigan). There were a couple biggins in Colorado that she bogged down a little, slowing down to 40ish, but that just allowed me to enjoy the scenery a little bit better. Enjoy your new truck!!