This web page shows the towing guides.
http://www.fleet.ford.com/towing-guides/4X4 2008 F-150 with 5.4L V8 and 3.73 rear end has a GCVWR of 15,000 pounds, unless you have the Harley Davison Edition or Lariat that have a more limited 11,000 GCVWR. SO you can tow about 9,000 pounds if only the driver is in the truck. Add 300 pounds of passengers, and you will start to lower your tow rating - pound for pound.
You also have a cargo rating too. Say your GVWR is 7,500 pounds and the truck empty is 5,500 pounds. This limits you to about 2,000 pounds of hitch weight, passengers and driver.
A average weight on the hitch of a 8,400 pound trailer will be around 850 - 1,20 pounds. Much depends on how you load the cargo, how far back the axle is from the hitch (or from the rear of the trailer) and where the fresh water tank is located, and if that 60 gallon (480 pound) tank is full or not.
You should be fine.
Fred.