Probably just tread squirm with the new tires, but these guys seem to believe there is better sway control to be had. The BO favors center like a pendulum but offers no friction to prevent sway from starting. I had sway like you describe with my Reese HP without the sway control arms installed. Not too bad but could be annoying with lines of trucks passing. Then I picked up a barely used Equalizer 4pt 10k off of Ebay for a song. Solid as a rock now even with crosswinds and trucks passing. Our Jayco is a little shorter but both have a long wheelbase and tow great without sway control. For the little bit of wind or road induced sway, a little friction will stop it.
https://www.etrailer.com/question-116001.html