I'm on my third stock diesel truck (duramax/allison) carrying the same Lance 1071 camper. The '03 xcab 2500 got 9-9.5 mpg loaded. The 2005 xcab 2500, got 9.5-10.5 with the same load. In 2006 I drove it unloaded to LA from Portland, cruise on speed limit and got 20 mpg. Return trip, with 1500 lbs, at speed limit plus 5mph got 18.
The 2010 crewcab 3500 gets 13-15 in town, 9.5-10.3 loaded with the camper. The variance in mpg loaded, in my opinion, is due to the effect of which way the wind is blowing...tailwind= 10.3, headwind 9.5 or so. Not really exact science, just my observations.
Oh, power has never been a problem with any of these trucks!
ps. Just read sleepy's previous comment.....he's right....wish I had bought the crew cab 3500 from the start. I'd still be driving it!
See you out there,