Marine grade plywood is best. If it's only a 2 place trailer, you're only looking at 2-3 sheets of plywood (8-10 foot).
Exterior grade plywood would be my second choice, and you can use a wood treater over the top of it. If you want to be crazy you could do both sides of the plywood and the edges all the way around.
I also used stainless steel screws to hold it down.
FRP would get stiff and brittle in the winter, might crack or warp in the summer. Also very slippery, in general. Might not be the best solution for a decking material. Just my opinion....
BTW, might also want to re-wire the trailer, at the same time. I would imagine the wiring would be pretty easy with no wood deck on there????? Not a bad weekend project - only a 12 pack job