The problems arise when the water is trapped and not allowed to dry out. Parking on a hill would help, and keeping the van dry would also help. The thing is when you have things fitted like tanks or wooden floors the water gets trapped underneath and will cause damage over time. If anyone is using a paint for protection I would advise the removal of the wooden floor if any.
The outside of the van will rust over time as will the chasis. Both are allowed to dry out after they get wet, but they will eventually rust.
Peter