anonomoose - I forgot the newer pressure treated is ACQ and is far more corrosive to aluminum. Adhesive roof stripping or gutter patch would work great on the trailer cross members to prevent contact.
zimmbob - Good idea to waterproof the bottom of the wood right away.
thunderstuck88 - The problem with plastic / PVC is that it will be way too slippery. Most of the decking requires 12-16" spans so I would need to add more aluminum framing to the trailer. It was also 3x the price of wood when I priced out the deck on my house.
snobuilder & snolover55- I looked at the trailer and I think its 1/2" thick?
twark_kesslers - I think if we stored this trailer in your garage we would have less maintenance! Let me know when I can bring it over.
I'm going to look at the trailer again to see if we can make it through this season. When we are ready I think we are going to use 1/2" thick marine grade @ $50 per sheet x 6 sheets. We will buy it in spring and let it dry out for a few months so we can apply a waterproof sealer to the bottom.