2012 Expedition EL w/HD tow package, longest travel trailer?

You will be limited by either that 890 pounds of receiver or payload, not length. 890 pounds of TW is near 5500 pounds dry, 6500 loaded for a long weekend of family camping.

There are many people towing 35+ long TTs with similar wheelbase vehicles. The drivers who are comfortable do it are doing it with 3/4 ton Suburbans, 1400 pound TW limited receivers, and over 2000 pounds of payload.