Here is an option to POR 15.
http://www.eastwood.com/paints/rust-solutions.htmlEastwood I see now also sells POR 15... They changed their web site over the years and use to have several technical papers Rust Encapsulator verses POR 15. I cannot seem to find that right now.
I have the Eastwood rust converter and the Rust Encapsulator. The converter for hard places to even attempt to get a sander on. And will chase the last coat with Rustoleum Professional paint. I do not want to do this again… It is a spring time project, I missed this year, Too busy.
Some just use the Rust Encapsulator. All depends on how many years you want to keep the camper.
Whatever product you pick, read all the directions, some need to be applied to bare metal, some are not UV stable and need a top coat. Others need primers.
Good luck, this is a painful job crawling under to go and a real good one to do to help save the camper for long term. Do it right with whatever product you pick or you can do it over again is several years. Again back to how long are you going to keep the camper?
As FYI Jeg’s High Performance also sells Eastwood products. I’m sure other auto restoration places do too. Or buy it direct.
Good luck
John