AFAIK the susceptible bits are wax treated in the factory, I've certainly come across wax in the bottoms of the doors & cills. That said, I have treated bits of our van with
Metect, I used to used it professionally before I retired & it's my goto rust preventative. I've used it on boat trailers that regularly get dunked in the sea and on bare steel stock that's been stored outside. We used it on sewage handling machinery, and that's about as a harsh an environment as it gets.