Plywood & Rubber mat question ( Who's on first )

Seal plywood first - keep out the water, which will eventually soften the plywood, or use green-board (waterproof board used for outside ...

Then, put mat under the plywood and it won't slip.

To save weight, and plywood hassle, I cut four pieces of 1x6 waterproof decking, and ran it at each end and two in the middle. Screwed it directly to the floor of the camper. Works great, and allows ventilation under the camper so water doesn't settle with no way out.