I wonder if sometimes these sway questions come from people that are inexperienced or bad drives. Not in the OP's post of course. He is just telling us what he saw.
I tow a 20 foot TT with my half ton pickup and use no weight distribution hitch or sway control of any kind. It tows quite nicely. I've been towing trailers for 35 years.
I was on a 4 lane road the other day and got passed by a semi and I see what you are all talking about with the suction of the semi truck pulling you. I just held the wheel straight and kept me speed stable and let him pass. It was not a white knuckle experience at all. Your dragging the trailer, don't let it drag you.
Don't over compensate, if fact, don't compensate at all. If you go straight the trailer will too. You have to pull it, not let it pull you.
Now, if I was going cross country in mountains with high winds I would give a some of these sway control devices a though. As a weekend camper that lives on flat ground and never goes more then 70 miles from home I sometimes wonder if some of you maybe just need to learn how to drive. I have no experience in other places though and maybe that is why I feel that way.
But, Still, just drag the darned thing and get it over with.