Taken all the steps to prevent sway. Still swaying. Help!

You will always feel a little "push" from a big truck passing. That's just physics. But it will be very quick and you should quickly feel the trailer straighten out if you have a good setup. It is definitely not a white knuckle feel.

One thing that helps with the big truck effect is to simply slow down. If you are running 65, drop it back a couple of MPH and you will see an improvement.