Does this sway happen when no other traffic is around and no wind? Tread squirm and light suspension can cause your rig to waller around in your lane. If it only happens when other traffic is passing or mild crosswinds, but then straightens out, it’s likely normal reaction with a lightweight truck. I tow our 32.5’ TT with no sway control with our F250. Tows like a champ but we occasionally feel passing trucks. Box trucks are the worst as bow wave pushes while vacuum pulls the rear as they pass. Nothing scary but it’ll keep you awake. If you’re having sway as in oscillation above 45mph with no wind or traffic around, then I’d say tongue weight or too much hanging from the rear of the trailer. Some trailers tow better than others as well. Short wheelbase trailers are worse. Too much WD Trying to compensate for not enough truck can induce some steering disturbances as well. Been there too many years ago.