The walls are paneled with 4' x 8' x 1/4" birch plywood.
Above that is a 1" x 6" pine board -- the 1/4" birch overlaps it by about 2 inches.
Immediately above the pine is a half of a 4' x 8' x 1/8" birch plywood panel (ripped lengthwise). When this is done on both sides, the remaining space in the center is 4' wide -- perfect for a whole 4' x 8' x 1/8" birch panel (added after photo taken!). No waste and very little cutting.
Oak trim strips cover the lengthwise junctions between the panels. We used almond-colored panel moulding to join the "short" joints.

The supports are to allow the glue between the paneling and trim to set without gaps forming.

We mounted our new folding steps from Step-Up Manufacturing (link). They are constructed of diamond-plate aluminum, and weigh only 30 lbs.

It's a high-quality piece! They're adjustable for off-camber parking situations where the distance to the ground may vary.

Folded steps. The mounting bracket (lower right of photo) allows them to be mounted or removed as needed.