First off let me say I am not an engineer and I could't explain the math to you even with a slide rule. That said.., not including doubles and tripples, the idea of a 5th wheel hitch having a sway issue.., well I just can't see it.., as you have said. The pivot point of a 5th wheel hitch, that makes ALL of the difference. At that point there really is no possibility of sway. The pivot point is usually directly over the axle. Contrast that with the pivot point on a conventional hitch being behind the axle. At that point the towed vehicle has leverage on the towing vehicle and that is amplified by the length of the overhang behind the the axle. You never have that issue with a 5th wheel hitch.
The sway you see with doubles and tripples occurs because these vehicles are coupled similarly to a conventional hitch (pintle hitch). Pivot point on tow dolly behind axle on preceding vehicle. Multiply this by two hitch points (with tripples) and the problem is compounded.
A 5th wheel trailer by its design cannot sway as we understand swaying again because the pivot point is usually directly over the center line of the rear axle of the TV. With semi's the added advantage is that they have adjustable hitches where the hitch can be moved forward. I know that is usually associated with loading but the idea of the hitch forward of the center line of the rear axle I think helps to see how sway would be, I dare to say, impossible provided all things being equal. I'm not speaking about the unseen circumstance but even with that I can't see it.
As for sway control, it is going to depend on each individual setup. You can't throw a blanket over it and say it covers it all either way. Some will need it and some will not. That said, as the seat belt and ABS ideas were floated. It does not hurt to have it and it is better to have it and not need it vs. needing it and not having it. I would hate to be in the latter.
