If it’s wallering around, I’ll agree on soft tires or suspension. If it starts a rhythmic fishtail around 60 mph, that’s light tongue weight or more accurately, too much weight aft of the axles. Rear kitchens or rear cargo carriers can quickly throw things out of range. Our travel trailer barely have enough tongue weight empty so they can be advertised as towable by lighter tow vehicles. On some smaller trailers, as little as 100 lbs added behind the axle can create that kind of oscillating sway. I like to set up my hitch so the trailer is slightly nose down. Never nose up. My loading rule is, load anything you can carry as long as it’s placed over or forward of the axles. This rules out a rear kitchen for us. I use as little tension on the WD bars as I can to bring the front about halfway back down to unhitched height. Too much tension on WD bars can cause steering disturbances as well. Minus passing vehicles or strong crosswinds, you should feel no sway even without sway control.