If you look you'll see lots of info on fuel economy. I think I'm pretty normal with my 2012 6.2, Ext Cab, 8' bed, 4x4 and 32' 6,800lb bumper pull.
Hwy empty: up to 17 (on a good day w/ a tail wind!)
empty city/hwy mixed: 11-13 depending upon the mix
towing: 7-9 (most common is 7.9 to 8.2. Lowest ever 7.0, highest 8.9)
For my needs the 6.2 is outstanding. The transmission Ford hooked it to really makes everything smooth and strong.
In the Rockies though, my trailer pulls the truck down to 2nd on many hills. I've still got good torque, so I'm not floored, but I typically can't hold 3d gear.
I would be concerned about that much weight (your 11.9K 5'er) behind the gas motor. I like mine a lot, it's a good match up, but you're talking an extra 5,000lbs.
Scott.n.Tira from Las Vegas
2021 GD Imagine 2670MK
2012 F250 4x4 6.2 LB