With my first Class A I spotted lots of rust underneath, crawled under there with a spray bottle filled with Jasco Prep & Prime. Sprayed wherever I found rust, avoiding rotating objects. This stuff is an acid that chemically changed the rust back into metal. Inside some of the basement compartments, big flakes of rust were obvious. Removed those flakes and painted the other surfaces with the Jasco using a foam brush. Surprisingly never had to worry about that area afterwards.
Worked like a charm. Over the 12 years of ownership of that rusty rig, never found any additional rusting, and never had a mechanical issue I could attribute to rust. I periodically checked under there as part of yearly maintenance but never had much more to do. The rust never got any worse, and in most places, it was a nice shade of black instead of rust colored.
My next Class A also has rust (though not as bad) and I did that same treatment. Saves a lot of time over grinding and wire brushing as it is very easy to apply using either a spray bottle or a foam brush.
Home depot now has a less expensive solution:
Klean-Strip
One thing I noticed is that the product would eat my clothes up if I got in the wrong place while spraying or brushing it on. Always wore work clothes, face mask, and goggles.
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