On my 19 I had lots of issues with frame rust coming from the battery mount. This was before the storage unit became available. I choose to go over everything with a wire brush on a drill, a coat of Rustoleum primer and then their semi-gloss. After one trip you could not tell anything had been painted.
On units with an exposed battery box check the tray and mounting bolts.
Paul and Janet Braun
2003 Toyota 4Runner V8 now 2012 Toyota Sequoia V8
Escape 19' 2010 now 2014 Escape 21'