Your best bet is Allum. or Galvi for sure. Is that trailer Boxed in? Some of the steel trailers that I have seen like Norts AKA PhantomChaos is boxed in and preasureized. That helps out a lot. The problem with steel boxed in trailers is that they salt water gets on the inside untreated part of the tube. Then starts to rust from the inside out. The outside of the trailer can look great but the inside can be totally waisted. Then you trailer breaks in half. I have seen it happen!!! If you can wash the entire trailer down both inside and out then steel is ok otherwise I would opt for the allum or galvi trailer.