Most home improvement stores carry several rust abatement products in the paint departments. I use Jasco from Home Depot, comes in a quart bottle as a liquid. Put it in a spray bottle and go to town on the rust underneath. It's an acid that chemically changes the rust back into metal. Turns it black in most cases. Stabilizes it so you can paint it if you want, I never bothered. I've used it on two RVs now over 13 years of full timing. One had big flakes of rust...chipped those off, than sprayed. A friend used a foam brush instead and her undercarriage looks better than mine. I'm jealous.
But the point is that the metal underneath is generally so thick, it would take a century for it to rust though in most areas. The acid treatment slows it to an imperceptible crawl.
My '02 Journey was actually in better shape then my '94 Bounder rust wise, and I treated them the same with the Jasco, and then never worried about it again, BUT, I don't drive in the east with their salted roads, either.
