If you want a diesel, get a diesel. I did and do not regret it. Might I suggest you spend some time on
www.dieselplace.com to get a feel for the various versions of Duramax out there and experiences from those actually driving them. Mine is the 2008 LMM which has the diesel particulate filter but no urea treatment. Average 14mpg highway with the camper in my sig and climbs mountain passes without breaking a sweat. Not to mention goes down mountain passes barely touching the brakes. If your comfortable running an engine at the high RPM's a gasser will need on mountain roads, than a gas engine is certainly an option. I went with a diesel because overkill is underrated.
(edit)PS: Count me as another vote recommending you go right to 3500 if possible.