Originally Posted by barmcd
I'd say yes. A few years back we sold our house and lived in an RV park for about a year while we built another. We had a long gravel parking pad and I parked my previously garage kept HD motorcycle on the pad. I kept it covered, but fought rust for the entire year as water would condense on the bike under the cover. I'm sure the underside of the coach is the same type of environment.
Your cover was waterproof, but not air-tight. Moisture trapped under the cover stayed because it couldn't evaporate as quickly as it would in open air. The humidity went up and rust happened.
If the OP puts down plastic, it will also trap moisture under it. The additional water will create a 'mini-marine' environment around the edges as the moisture escapes. This added humidity will accelerate rust, not inhibit it.
Most of the rust pictured will cause little damage to the RV for many years. Perhaps a misting of oil on the gear rack and bolts will slow down the rusting, but unless he moves to an arid area, rust will continue, even behind undercoating.