You don't need exposure to salt for rust to form on the undercarriage of an RV, or anything else for that matter.
Park a car in a field, or on a grass surface such as a lawn for a few months and then look at the undercarriage. You'll see a lot of surface rust forming on pretty much every weld/seam. And it doesn't take long. Ground/dirt creates a very humid environment, and will grow surface rust very quickly.
Knock off the flakey stuff with a wire brush and hit the area with POR-15, using their surface prep first. You could probably use any other rust converter product as well, such as Rustoleum's converter product that comes in a rattle can.
Jim
