How large of a family do you plan on carrying while towing? In all reality, you shouldn't have any issues. With my family of 7, I have enough payload for just under 600# of tongue weight.
Have you driven a new Expedition EL? What about a Suburban or Yukon XL?
I have driven three different Expedition ELs as rental vehicles, and while the comfort features are nice and it has good acceleration with the EcoBoost, in my opinion the handling is poor. These were all 2wd models. To me, it has a very poor on center feel and it feels like you are wandering all over your lane/the road. Maybe the steering is too sensitive? One of my coworkers noticed the same thing.
I have also driven a new Tahoe as a rental vehicle and like how it drives much better.
The GM 5.3L will also do a fine job of towing a tent camper.
On my last business trip a coworker had a Suburban and I had an Expedition EL. When following him around on both side roads and on the expressway, for the added HP and torque the Expedition was supposed to have, I was into the gas way more than I thought I would be to keep up with him.