Not exactly related to your question but.........
The more I think about it I guess it really depends on where you are.
When I lived on the West Coast on Vancouver Island rain didn't hurt anything. OK you might see moss growing on a B van that wasn't used but rust wasn't a real concern. Lawn mower didn't rust after a decade etc.
On the East Coast the salt is a killer as well ground moisture or humidity. Rain isn't a cause of concern - no rust on my lawn mower etc. (kept in a shed) but if you left a vehicle outside on gravel or grass it would deteriorate much quicker from what I've seen.
I lived in a subtropical climate (Bermuda) and everything outside could rust from exposure to salt in the air.
PS. I really did see moss growing on a Roadtrek on a dealer lot