Pulling my old '97 flat front profile 11200 lb 5th wheel rv trailer with the '03 2500 Dodge/Cummin NV5600 305 hp/555 torque with a same year Edge box for towing non adjustable bumps hp to around 350 and 650 torque.
I've noticed better mpg (10.5-11.5) in CO/NM/UT/WY/MT mountains than in high winds in OK/KS/TX/NB flat lands (9.5-10 mpg).
Running empty on long trips will average 21.5-22 mpg at legal speed limits.
I ran the truck as my DD for the first 40k miles when new. My commute was a 660 mile a week average all county roads and state hiways and interstate ...no city driving out here. The Cummins would average 20.5 --21.5 mpg week after week.
My style is different than some. I've RVed with station wagons...panel trucks...several truck campers... then some type of RV trailer since the early '60s and have always been interested in getting the best mpg with my equipment. Many times no bucks for fuel or cost of going on a vacation.
Some folks don't have big pockets to buy anything new or the newest or go anywhere they want at that time.