I have an 18ft car hauler with a wooden deck. I have kept it stained for the past ten years with whatever I had left over from staining my deck.
My deck is almost 20 years old and I need another few years before we redo the back yard. So I tried this stuff:
Restore Concrete & Wood Deck Sealer by Rust Oleum - Essential Hardware
Worked great.
I would, after a through pressure wash, try it on my trailer.