looking for ideas (recovering camper from mud)

The problem with wood/plywood is once you get mud on the tires you will have no traction on the wood. Unless you attach some furring strips across the boards for the tires to grip on you will likely just slide off of them. Best would be to install them in a "V" shape with the bottom of the "V" pointing away from where you are going. That way if the tire slips it would slip towards the center of the board.