Plywood, 1/2" thickness, about 6" of overlap when pulled out.
The main source of strength is the edge or pull out piece (facing the main part of the camper). It is a 1x4 (actual measurement is 3/4" x 3.5") by the entire width of the camper made of hard wood, mine is oak. It has a dado about 3/4" up from the bottom that allows the plywood to fit in and make the unsupported edge very strong when pulled out. I believe that the plywood and oak dado are only glued together as I can see no nails or screws to hold it in place.
I removed mine when I initially got my camper and ran the forward edge of the plywood though my table saw, with the blade tilted at a 45 degree angle to create a beveled edge, instead of just the squared off end. I also sanded the now angled edge really smooth, sanded a bevel on the sharp bottom edge or acute angled edge as well, and it slides forward into the stored position a lot easier now.
Oh yeah, if your interior side panels have the screws at the top, you may want to pull back the top edge of the interior side panels and measure how far down from the top the screws should be to make a good connection with the aluminium frame for holding the side slider hardwood pieces. Also take a look around for electrical wires and if needed secure them out of the way from where you are going to drill holes for the screws, use some gorilla tape or other long lasting type of adhesive tape. My side panels came with screws about 10" apart from each other, however, I put some additional screws between them, for insurance.
Here's a photo of the extra aluminium stringer that is usually installed in the frame for mounting the side sliders.
Here's the placement of my screws, about 5" apart.
And, don't forget to put some kind of a "stop" at the rear end of the side slider to prevent the pull out from working it's way any farther rearward while you're sleeping.
Here's a good photo of the dado in the hardwood edge of the plywood.
That should be able to help you out.