I've had my 15 2500 4x4 with a 6.4 for about a year and a half now, and I'm pretty happy with it. Previously, I had an 08 with the 6.7 Cummins.

My 6.4 will get 17+ MPG on the highway at 70, it falls off going much faster than that. Towing an open trailer with a car it seems to be 12-13 MPG. I've never seen single digits, but I typically pull 6-7000 lbs. The MDS helps the 6.4, my truck gets better mileage than my friends 14 5.7.