I would use PT plywood. If your trailer is aluminum you could always put a barrier between the frame and the plywood. If you know anyone that has any extra Ice and water shield lying around you could cut strips of that and stick it to the frame before mounting the plywood.
I've done this with decking. I put strips on the floor joists before putting down the decking. It keeps the tops of the floor joists from rotting because of water being trapped between the joists and the decking. It also prevents squeaks.
That Ice and water shield is like Franks Red Hot, I put that ***** on everything!
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