The floors of caravans are manufactured from exterior grades of plywood which use glues that won't absorb moisture therefore they don't require any further protection.
It is only the ply laminates which are "glued"and the black coat is preservative which over time does wear away leaving the surface open to the elements. I have sprayed mine in places where this has happened. After all its only ply and not something special apart from being a waterproof ply( i hope).
it is usually pressure treated marine rated ply. nothing should ever be used to underseal. it will stop the timber from breathing and will cause the timber to sweat and rot.