OP, the suspension on nearly all bumper-pull RVs and 5th wheels is state of the art directly from the 1920's. VERY rudimentary. It's one of the major causes of tire failure.
- pics would help with suggestions as has been mentioned. These suspensions lack two major things: dampening of vertical movement and the prevention of lateral movement. Something like a panhard bar and a set of shocks can rectify this, but only if you have appropriate mounting points.
- Tires, while a great upgrade, won't address either of the above items.
Good luck! Update with what you're doing.