My dad's first trailer in the early 70's had sway issues. It was not a toy hauler and was fairly small, about 19' but it had tandem axles (which at the time was not seen much on a small trailer). Had no gray tank and had a small black tank. Small water tank in the front. We had to move all weight forward, canned goods, fire wood, always drove with a full fresh water tank. The most weight we could get on the tongue was about 350 lbs. Not much. The real issue was the trailer was built with the axles too far forward by an inch or two.
Weekend Warriors were known for very heavy tongues as well as very heavy trailer themselves, well beyond published weights. Some of the longer WW TT went from dual axle to triple axle to offset tongue weight because it was so high.
It might be axle placement might be a little too far forward on this trailer. However, I'd work with everything else first before giving up.
