I'm towing a 5'er at about 8500 lbs loaded. The 2013 F150 Platinum Ecobeast truck with towing package has 29000 miles on it, of which 5500 has been hooked to the camper.
At 55-60 mph, I get an easy 10 mpg pulling in the mountains, just made a 1055 mile trip to Shenandoah National Park two weeks ago. Around town 17 mpg, and 21 empty on long trips. Pulling at interstate speeds loaded, it will drop to 8.5-9.5 mpg.
I have a towing tune installed (just to mess with the V8 guys 😉
which punches it out to 415 HP and 435 lb ft torque.
I bought it with 6000 miles on it for $40,500 a year and a half ago. It is great for how I use it. Like the OP, no way to justify the bigger, beefier trucks for that kind of use, even with me pulling probably a little more of the time than the average non-retired camper. We tend to make longer runs to our preferred campgrounds.
2015 Flagstaff 8528RKWS
2013 Ford F-150 Ecobeast