Ron...
These are a few factors that "I" know of that can create possible
trailer sway.
Reasons for sway caused by a trailer:
Tongue weight is too little OR too much
Your center of gravity might very well be too high
Tongue length is too short for the over-all wheelbase
Trailer suspension is too soft...common problem with
added weight
Trailer frame is too flexible.... this last one is the most common
cause once extra weight is added
And reasons of sway that can be caused from your tow vehicle:
Wheelbase is too short
Rear overhang is too long
Rear suspension is too soft.... weak springs
Rear-end weight is too much...vehicle overloaded
Rear cornering stiffness is too low... lack of a good swaybar
Also under inflated or worn tires can create the same problems,
even broken cords in the tires...especially if they are bias-plied
A sway bar can help, but it's just a band-aid.... find the problem
and remedy it, instead of trying to cover it up
just my $.02
I've got a few miles towing things under my belt