I towed with a 1998 and a 2001 Expy. Both 2wd and 3.31 axle with a 5.4. Towed a 25 ft TT right at 5500 lbs. Like previous posters have said 8-9 miles to the gallon in 3rd gear. Just let it rev, sit back and go. The 5.4 will pull your TT just fine. If possible do get one with the 3.73, it will pull better yet.
We currently have a 2004 Expy with 245,000 miles on it. Runs great. Just have the truck looked over well, change all fluids and anything else you feel needs attention. The Expedition is an awesome vehicle. IMHO.
Happy camping, John