Gas engine fuel mpg?

We have a 2016 Ram 2500HD, 2WD Crew Cab with the 6.4L Hemi. In 2017 we traveled from Georgia to British Columbia and Alberta and back stopping at Glacier and Yellowstone National Parks. We were carrying a fairly light Adventurer 80RB coming in around 2,400 lbs as loaded. Covered about 8,650 miles and averaged 13.8 mpg hand calculated. Over one stretch of 300 miles with a tail wind we achieved 20.2 mpg while the lowest daily average was 12.8. This was driving sedately between 55 and 65 most often with the occasional 70 mph for short periods of time.