I've owned A 2016 F150 XLT screw with the short box towing a 6000LB (when loaded 28' TT and once I found the right hitch set up and used Bilsteins in the rear , it towed beautifully
tried 2 different WD systems and finally got it dialed in with the Reese Dual cam system , did a 6000km trip on all types of roads , grades , weather and no issues.
I now own a 2018 F150 XLT screw with the 6.5' bed and now tow a 34' 5er and only issue i have is a little bounce in the front , but it tows and handles this new unit perfect
maybe that steering issue is the culprit