Many years ago, I had an MG-1100 sedan with what was called "Hydrolastic" suspension. A truck-sized version would be great for RVs.
There were no metal springs holding the car up. Instead, there was a rubber spherical device at each wheel location that was filled with a non-freezing liquid. The spheres were interconnected diagonally from front to rear by metal hydraulic lines with restrictor orifices. The system offered a fairly soft vertical ride with a high transverse (roll) stiffness and also kept the vehicle level fore/aft regardless of what you drove over.
It went out of production in the 1970's, but it might be adaptable to RVs if a manufacturer wanted to try it.
I think it was used in the Austin "America" and Austin 1800 and Princess sedans also.
