Rust bubbles

[cattfuu]

Continuing our search for a riveted 1997 to 2000 coach we stumbled onto a contender that fits our requirements regarding lay-out and availability of manuals, but we realized rust bubbling under paint. We were told that all 1997 buses will have rust bubbling on them and that this is no big deal. Made the hair stand up on the back of my neck as I know that if you end up with rust on a car that comes from the inside out, that means the damage behind the panel is much worse. All the rust bubbles were at the bottom edges of the upper panels under windows and the front wind shield. We were assured that the window gaskets were in excellent shape and that there were no leaks. Tell me that I am wrong in my assumption that in order for rust to form there has to be trapped moisture behind the panels which tells me there are areas where water enters. If not from around the seals then from where? To us this looks like a major maintenance nightmare/expense and we think it's best to keep looking.
What say you?