The 5.4 is not a particularly strong engine for towing. My guess goes with 8-9, maybe 10 on reasonable terrains. The biggest factor is probably going to be how fast you drive. ST tires are only rated for 65 MPH, so 60 is a reasonable towing speed and gives you some wiggle room. That's where I do best and I'm towing an 8000lb trailer with a 1/2 ton. No, I don't have the same trailer in my signature, but I still lurk around all the forums.
2013 F250 XLT
I now have a Keystone Outback, but I try to help when I can.
Bitter Gun Owner