Yep,
Without pictures showing just what you're concerned about and, what's showing, it's pretty hard to help you with your issue here. It's also pretty hard to imagine "Rust" of any type, on any ROOF on any RV out there because, primarily, there's really no RV roofs out there that have steel or metal roofs that will rust. There's aluminum roofs but, they don't rust. There's rubber roofs but, they don't rust. And, there's fiberglass roofs and they surely don't rust.
So, this is a new one on me, that's for sure. Pics would be a good thing here.
Scott