99% of the time when people talk about "true" sway it is because of lack of tongue weight.
Water is heavy. Some trailers are of poor design. The further the fresh water tank, black or gray tanks are away from the axels the more potential for screwing up the weights and balances of the trailer. Some trailers will change dramatically by taking water out of one tank and putting it in another. (Fresh to black....etc.)
Trailers that are right on the border of sway are very particular in the way they are loaded. I know because I have one. If I load heavy stuff in the back I can turn a well handling trailer into a nightmare trailer. Just a few 100 lbs can make a BIG difference with some trailers.
Check the way the trailer is loaded. Both with water and things.