Some misinformation here. The inner layer of the outer walls is plywood and the structure is aluminum, not steel. The foam is closed cell so it won't absorb water, but water can saturate the plywood and compromise the adhesion of the wall lamination. Here is a construction video to showing the wall assembly.

i added a deck access hatch to reach the water pump and I hd to cut a hole. Looks like the walls and steel frame and foam composite. No wood found although the Styrofoam can mold if it gets wet so stay up on your lap sealant checks. If your adding a cover access the wall size is 1.25inches.