1 you can laminate over pinitrating epoxy just scuff sand the aera uith 36 grit sandpaper.
2make sure that any aeras are CLEAN before you try to laminate cloth over them,
3 the only differance between marine grade fir ply and exterior is that there are no voids in the innerlayera(ie knotholes ect.)As far as rot resintance fir is about as good as you can getEpoxy coat all serfaces paying close attention to the edges as most dampness absorbed bt capilary action onthe edge grain.
I would slightly tilt the trailer in all directions and pour epoxy along the channel so it can be sucked into the end grain. eather pinitrating or regular epoxy.
Jim Smith