Calling Duramax owners 1st trip with camper bad fuel mileage

We have a 2012 GMC with which we pull a Jayco HT 26.5 RLS and a 19' Crestliner and get about 12 mpg on all of our trips to the lake to fish and relax, about 150 miles away up over a pass and then back down to the lake, and reverse on the way back. Took our FW only on a 3,800 trip two springs ago and got 12 mpg, hand-measured for the whole trip. You might have and injector problem because I even got that mileage with my 2003 GMC. Not so good with the 2011 Ram diesel that I had for about nine months.