I'm going to go with severely or not so severely, out of balanced tires.
Both of my Ford Dualies came without balancing on the inner rims. I can't remember if the outside rims were balanced or not.
After getting new tires, after hitting a big bolt, and finding them cupped, I had them all balanced. big improvement in ride. The rears still wear pretty fast.
Air bags? My Neighbor built a custom cross member and had air bags in addition on his F350 Dualie. His Fifth wheel was really heavy though. Shocks on rear do take a beating with 14,000 lbs and more on the rear. An air ride hitch helps a lot and keeps things on the shelves in the fifth wheel too.