You need to give us some additional information on your Expy.
What is the wheelbase ?? With a 27' trailer, you should have a minimum of around 135" wheelbase.
Do you have the Ford heavy duty trailer towing package, and what are the weights with that ??
A lower final drive ratio (higher number) is better for towing, so see what that is.
What is your hitch rated for?? There should be a label on your hitch with both weight carrying and weight distributing numbers.
The Sway Pro 550 might be a little lite for that trailer. The 255RS has a listed tongue weight of 500 lbs., but will probably be well over that by the time you load the trailer for camping. The 750 lb. unit might be a better fit.
Give us more info, and we can go from there.
Chap , DW Joy, and Fur Baby Sango
2006 Ford F150 Super Cab 4x4
2008 Surveyor 263