The ONLY time I have had fish tail sway issues, is a trailer that is loaded incorrectly, IE usually too little hitch wt! As noted, a heavier suspension truck will not solve the issue. My 12K empty IHC dump flatbed does not stop my 8500 lb equipment trailer from swaying if I have no hitch weight! Yes, been there, done that!
With that said, ASSUMING you do go with a 5w trailer, now you need to sorta decide the size 5W you want, hitch/pin weight of said trailer, then get a truck that has the payload to handle said trailers pin weight! If you are in the mid teen lb relm, I would swag, most any of the current motors will handle that weight, yes with some speed loss, watch the fuel guage go down really fast too! Main issue in my mind, is payload for the truck. Drive train that will get you to where you want to go with speed on freeway you want. Along with, if you off road towing per say, will it have gears low enough to get you up steeper grades etc you may run into!
I personally like a few others, do not believe you currently truly need a new truck......at this moment in time!
marty
