Would be good if you could post a photo or link to a picture of what you are going to build for those not familiar with your design. Then we could figure out the configuration and sizes.
It wouldn't make any sense to overbuild with thicker ply on a small lightweight trailer.
Will your walls be sandwich or simple ply walls?
My floor is 1/2" ply and my walls a sandwich of 1/8" ply, 1" styrene foam and 1/4" ply. It is rock solid. As in mobile thermonuclear shelter.
You will be surprised how strong the trailer will become as it comes together with even with lighter materials. You will hear engineering types here talk of the strength of the
monocoque.
Maybe one of our tame Engineers could offer some definitive advice?