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17 EL Limited and my toy hauler is a 2011 KZ MXT301. 35' long and GVWR is 8800#s. The Expedition will tow every bit of it. Unlike Chuck Ive used a traditional "cheap" bar & chain WD hitch with sliding sway bar for years and shes straight as an arrow running 70 down the hwy.
Since you have the SWB model I wouldn't recommend one as big as me and DF5.4 have. It'll be the tail wagging the dog no matter how good your hitch is or how good your breaks are. If I were you I'd stay in the 25' or less range. Not sure the size of your family as you only mention the wife. We're a family of 5 with me, my wife and 3 small children and we have a UTV and multiple ATV's so we needed something big. I purposely bought the EL just because of this very reason.
I just bought a tuner for my ride so I can get some more performance out of her especially a torque boost so I can tow more efficiently. Like DF5 above I plan to upgrade my tires to an E rated model at some point. The OEM Hankooks are good but you'll get a better ride out of the stouter tire when towing heavy. If I'm really conservative and stay between 65-70mph I can get 11-12 mpg. Good luck with whatever trailer you choose
PPS ...at least get some frickin tow mirrors you menace to society