Interesting discussion. After towing fifthwheels for over 15 years I am considering switching to a 42ft tag-axle Class A and the thought of dealing with crosswind's on a Class A is likely one of the biggest factors on the negative side of the decision equation.
The truck and fifthwheel combos seems to act as a hinged lever in the wind with 3 anchor point's to the road with the hinge point directly over the center anchor point restricting sway and causing the combo to act as one solid unit with 3 anchor points.
I got caught out on the I75 many years ago at 11pm one night with a 34ft fiver and a single rear wheel TV in a sudden wind storm with 60mph crosswind's. While I slowed to about 45mph along with the 18wheeler's it was not a white-knuckle drive at all until a stronger gust tore the awning clear off the side of the trailer.