I towed my 15K 5th wheel from MN to AZ starting with a little over 500 miles on the truck. With winter fuel and pretty cold until I got way south, I averaged 9.2 mpg with my signature truck. I got similar mpg with my 2005 3500 4x4 SRW with 4:10's pulling the same trailer.
Driving unloaded back in MN I averaged combined city/highway, right around 15 mpg. Now, with 2900 miles on the truck, my average combined mpg has slowly increased to 17 mpg. Temps have been in the mid 70 degrees so I may have reached my best mileage. Again, my 2005 got similar mileage here in AZ, unloaded.
So, my 2014 DRW is getting almost identical mileage as my stock 2005 SRW did but with significantly more power.
I should add that the computer on the 2005 always displayed about 3 mpg higher than actual hand calculated while the 2014 computer is almost dead on.