As a data point, I have a class C, which would wander on the highway and get pushed around a lot by semis passing. I installed big Hellwig swaybars and a Saf-T-Plus steering controller, plus got a good alignment, and these helped a ton. I then noticed that it felt wallowy and mushy (technical terms) going over undulating pavement, so I replaced the stock shocks with Bilstein HD shocks. This got rid of that wallowy feeling and I think improved the ride and handling a little bit.
So, for me the swaybars and steering controller made a very big difference and the shocks helped a little bit.
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