I have an 06 Dodge 3/4 ton 2WD Cummings and pull a Safari 25' FB. We just completed our 3 month trip pulling around 8,000 miles. We do not pay any attention to the overhead cousul as it is not accurate. Our mileage is determined by the miles driven between each fill-up. Our average was 14.9 mpg. I got as high as 16.5mpg pulling between Bakersfield and Merrysville, CA driving 55mpg. My wife keeps very accurate records of our fuel purchases and the mileage. Our mileage has not changed much during the last 4 vacations pulling our Airstream over 38 States.
Sometimes the mileage fluacated a lot and I attributed it to the quality of the diesel.
I couldn't be happiers with my Dodge. On the highway, not towing, and using cruise control, it is not uncommon to get 20-23 mph. No lie.