2016 Ram 2500 Diesel Crew Cab 4x4 Long Bed. Just did a 1400 mile round trip from CA to ID and back. Had 1000lbs of ATV and gear in the bed.
Lie o meter says I averaged 20.1 mpg for the whole trip. I'm betting it was more in the 18 to 19 range. Most of the trip was at 70 to 80 mph.
My 96 4x4 MT and 96 2x4 AT both return 18.5-19.5 in all conditions. I suspect if I did all 50-65 MPH highway I would break 20-21.