New truck, New trailer, TONS of sway!

op wrote:
I guess my original question in my first post was what might make it so squirrelly empty?

AS the forum says their are a multitude of things that can cause a certain combo to be squirrely.
We see folks with all sizes of trucks/bumper pull trailers with the same issues when their not set up right. What works for J. smith may not work for G. jones.

Would prowler sell such an unbalanced trailer?

You bet. The dealer is under no obligation to make your combo tow correctly.

Or can a poorly set up hitch cause that much sway?

Yes.
Looking around at different rv websites/TT forums your scenario is happens quite often.