Right, according to the Ford Towing Guide for 2013, the Expedition EL 4x4 with the heavy duty tow package is rated for 8,700 lbs. Subtract about 700 lbs from that for the weight of passengers, gear and trailer hitch carried in/on the tow vehicle and that leaves you about 8000 lbs gross weight for the trailer. Use its GVWR as your estimated weight - that way you can be confident you are not over-stressing the Expedition's capabilities.
The Expedition with tow package ought to handle that trailer very nicely - good choice!
Without the tow package, the Expedition EL 4x4 tow rating is only 6000 lbs, so you DO need the Heavy Duty Tow Package.