Ram 2500 Cummins the most fuel economical

I'm thinking it depends mainly on the driver technique and speed. I get 12 mpg +/- .5 mpg towing the fifth wheel in my signature with a 3/4" GMC 6.6 Duramax.

I'm very non-aggressive and tow at 60 mph on interstates and less than that on 2-lane.