Agree. 98% of wagging (low amplitude sway) can be corrected by getting a level hitch height and sufficient tongue weight (10-15% of gross trailer weight, with around 12% being a good target).
The fresh water tank is behind the
axle, and some people carry stuff on the rear receiver. Both tend to reduce tongue weight and must be compensated with weight forward.
The other 2% could be your driving style. Keep your eyes far ahead and avoid constant small steering corrections. Keep it smooth and steady.