I owned a 2010 F150 5.4 3.73 and towed a 31' 7300lb TT. Your F250 is heavier. Not sure how much though. Just an FYI, I was at max GVW with the F150 @7600lbs. so if your truck weighs that dry then use that as a barometer. I averaged 9mpg. I mostly towed in Oregon with a few trips to NoCal. I put about 10,000 miles on the truck towing. Final impression. The 5.4 is doggy, but rock solid reliable. Ford wouldn't put it in an HD truck if it wasn't. You will get anywhere you go without issues as far as actually getting there. But you will struggle with hills and mtns. I ran out of steam on one trip where we reached 6200'. NA motors lose a certain % of power at higher EL.
If you like the truck get it. I assume you're on a budget looking used. If it fits your needs then you should be able to make it work. I'll say again that don't expect it to do what you want it to without struggling. But IMO you should be able to run the crud out of it without any issues. FWIW when climbing grades the 5.4's sweet spot is 34-3500 rpms. Good luck.