Hey Trace,
Powder coating metal is one of the best coatings applied to metal framestructures we see RV's built on. If it's done right (Quality Control,proven production processes, simple manufacturer and dealer prep), we shouldn'tsee it fail for some time. All metal will begin to show the first signsof corrosion at some time. It's inevitable. What will eventuallycatch up to you is if you ignore it. You haven't and I applaud you!
Tricks at the dealership is to rattle can a black coatingto cover what little corrosion may appear. Makes them look pretty while it’s on the lot for sale. Normally though, it’s nothing to worry aboutif you catch it early. I’ll bet thereare a number of owners who haven’t looked underneath to see what’s there. Given an unknown, variable amount of time, itwill advance to a stage that may impair the structural integrity and potentialsafety of rig if you don’t keep an eye on it. Consider it an owners normal maintenance ‘to do’ list every year. At the very least, power washing the underframe after a long road trip during the winter (salted roads) is almost agiven.
I work for the Department of Defense as a PreservationSpecialist (they call me Rusty – Corrosion Prevention and Control Expert). I keep an eye on anything from M1 Abrams MainBattle Tanks, Howitzers, tool boxes, nuts, bolts and screws. There was a study done in 2003 that proved itcost the DoD $2.8 billion is losses due to corrosion. As a tax payer, that doesn’t make me happy. I spend all my time preventing it,controlling it and mitigating those effects. It’s true; an ounce of prevention is worth of pound of cure.
Rust-Oleum is a proven product. There are others that are better, but whatyou’ve done so far should get you by. Ifit looks like it’s getting out of hand, consider using a rust converterproduct. They’re expensive, but itshould keep you corrosion free for up to 10 years with little prep work and aweekend’s worth of your time. If you’dlike more information, PM me and I’ll share the details. I don’t want to endorse products/proceduresin the Forum.
Good luck.