Chris, I disagree. I have seen tons of seam rust start from the cracked
sealant in seams. The water hangs in those cracks and starts the rusting
process. My current Westy - multivan - yep, rusted rocker seam, all the way
through in several spots. Definitely started from the outside, no dampness
or rust evident from inside. I know of a Florida van with 69K miles, same
rust, same deal, no kitchen. Seam sealant cracks left that way for too long
will rust.
Karl