I think when my trailer was new I saw that there were some exposed ends to the siding so I sprayed them with heavy undercoating. The ends under the front cap were the same way. The sides are covered with the aluminum trim so I think they should be good. Not much you could do about it anyway...
If yours is not that far gone I would treat it with a wood preservative then spray with undercoating.
I drilled out a 1 1/4" hole in the FRP siding for a mod that I did and got a good chance to look at how it's put together. It's pretty tight did not look like OSB to me. Much more compressed than what you see at the lumber yard. Not sure what glues they used. Would be smart if it was marine grade but I doubt it..
Plywood or OSB if it gets wet your in trouble..