Jim@Hitek
What I've done to both of my Class A rigs with rusty undercarriages is stabilize it by spraying and brushing on Jasco Prep & Primer. It's inexpensive, Home Depot carries it in the paint department. It chemically changes the rust back into metal and halts it's progression. It can be painted afterwards but I don't bother. Just stabilizing it works fine.
Another discoverer of Jasco!:)
You can use the Jasco to remove oxidation in fiberglass, too.
I used to do fair amount of painting and Jasco was one of the products I used. There are stronger phosphoric acid based rust neutralizers but a user needs to careful with personal protection and over-spray. One of my favorites is Water Softener Cleaner, the green liquid in a bottle. It rivals some of the auto body etchants I've used and is a lot cheaper. I apply it, typically, with a garden sprayer or, for small jobs, with an empty Windex hand held bottle. Sometimes, I dilute it for just surface rust.