The ford 6.2 is a great engine. I towed our 26ft 8000lb gross trailer for two years with a 2012 F250 4x4 crewcab 7" box. Running around 100/110 KM/Hour or a little beter (60/65 mph) I would get anyhwere from 27L/100km to 33L/100km (7 to 9 mpg)depending on terraine. If I keep the speed to 90 to 100 Km/hr. I can keep it in the 9 to 10 MPG (23L/100) range. I live in Northern BC so lots of hills.
I now have a 2011 F350 4x4 crew cab long box with the 6.2 engine but have not towed with it yet. We shall see how it does.