I have a 2013 Expedition with the factory HD towing package and the 5.4 V8, 6 speed trans and 3.73 rear end. We pull a 30 ft Bullet Ultralite TT that weighs 5400 dry, 6000 loaded with a 600 lb tongue weight. It pulls it like a champ! We avg 60-62 MPH which is a safe speed, get 10 MPG like everyone else and with the E2 WD hitch, there is no sway!
MAKE SURE this Expy at Car Max has the HEAVY DUTY tow pkg . They also made a regular Expy tow pkg that did NOT have the trans cooler, bigger radiator and built in brake controller. It only had a hitch and a 5 prong plug in. You do not want it if it does not have the HD factory pkg!
Again, that weight sounds very high for a tiny 18 ft TT!? Is it all wood frame instead of aluminum? Get an Ultralight ( Bullets are fantastic!--check them out) and you won't have to settle for just an 18ft TT.