I'm glad I found this post.
I've been living in the "Cargo Trailer Conversion" section for the past 8 months. check out Gadget Mans Adventure Trailer.
I'm doing a 7x14 cargo trailer conversion and the floor is one of my next projects, right now the bottom is bare 3/4 inch dense chip press board.
The problem is, that what ever I do will have to be applied from the bottom up, since the trailer is already built.
I thought of some type of spray or roll on "black Stuff" ...??? yuck.
But I like Martaingod's idea of using the plastic sign board cardboard and screwing it to the bottom frame, but I'm sure it's pretty expensive for over 100 sq feet.
But I also liked Constrictor's story about how after 8 years, he see's no excessive wear to the underside of his wooden floor. ?. I've always wondered.???
Right now, I think I'll just paint the wood with some quality house paint. I'm sure I have a gallon or two laying around. who cares what color it is....

Gadget Man