First, it would be interesting to know where your unit was built and where your dealer is located. Then, what route was used to get to your dealer from the factory.
Given when your unit was delivered my guess is that the rust is being caused by ROAD SALTS. I had this problem with a diesel pusher and it was clear that the dealer had NOT power washed the undercarriage as stipulated by Freightliner for winter deliveries. Eventually, the MH manufacturer and the dealer split the cost of doing a clean up.
Here is a horrendous example of rust. The owner of this unit had a firm of metallurgists prepare an analysis and it dug up the road salts used on the highways used for delivery to the dealer. That salt coupled with a stray battery current resulted in galvanization. His settlement with the dealer and the manufacturer is confidential of course. Mine was no where near as bad as bbk3's. I can't imagine your rust is anything like this either.
2005 Winnebago Vectra Rust