Towing involves so many different factors. It is the whole setup instead of just one thing. For example, work you are getting done in exchange of your gas money is moving of air. Most of the engine work goes toward moving air out of the way of the TV and TT. Because of that aerodynamic of TT (low wedge shape vs tall flat front ) matters. I understand that SAE is working on new towing capacity standard that will also specify frontal area. I think your setup is fine. One thing that may be not to your advantage is the fact your Expedition is EL version. That means the distance between your trailer hitch ball and your back tires are longer than typical Expedition.
Swaying happens when the trailer is moved materally by external force ( typically cross wind or positive/negative pressure of passing vehicle). Back wheels of the trailer is used as a pivot and front of the trailer is moved side way. This side force act on the trailer ball and will try to move your back wheel side way. When the weight is right on top of back wheel like a fifth wheel, there is no lever arm and it is very very difficult to sway the TV. Bigger that lever arm is, easier it will be to move the back wheels.
I am not saying your setup has an issue at all. Most likely it is more than fine since I see plenty of Expeditions with long trailers at campgrounds. I am saying that maybe one area EL version of Expedition is not as good as other TV. If swaying happens, you can either increase tung weight as long as added tung weight does not exceed GVW limit. Or properly tuned anti sway bar help too. Or you can slow down.
Another factor is how comfortable you are about feeling the movement of traler. Do you expect to feel like you are towing nothing at all? Unless you tow with Crew Cab F350 Turbo Diesel, you will feel something. Will that freak you out and make you tired in long drive? My previous TV was bit tall and I felt too much of roll motion. I don't think it was unstable but it made me feel uneasy and tired on long drive. I got a new TV that is 6 inches lower and with better suspension.
Bottom line is that most important thing is good setup. Get to know good RV mechanic and make sure you get a setup you are comfortable with.
