Here are my stats for my 2015 Ram 6.4 w/3.73 gears carrying a 2,400# Lance 815 (7" narrower than most TCs). All data is hand calc and recorded in an excel spreadsheet. Always fill up at the start and end of a trip so the mileage is actual for the trip though does include the 20 minutes to run the camper to the storage yard and unload. Interstate is usually 64mph, backroads about 55mph with the mix being about 70% backroads. Empty mileage is almost exclusive the 15 miles one way to work through about 10 traffic lights in rural city setting.
Gals Miles MPG
Camper Miles: 50,046
Camper MPG: 12.40
w/trl miles: 4,256
w/trl MPG: 11.68
Mix miles: 9,208
Mix MPG: 13.55
Empty miles: 24,202
Empty MPG: 14.30
Total Miles: 87,713
Total MPG: 12.95
I usually do not use Tow/Haul mode because it raises the shift points and locks out 6th gear (double overdrive). The time or two I've used T/H for entire tank because of headwinds and slowing down to 55mph, I only lost 1mpg.
When doing my purchase analysis which inncluded decision on diesel vs gas, trip cost, overall operating cost etc, I only used 10mpg so I'm well pleased with the mpgs.