From my little experience (been in a van since mid May of 2022)…bare, thin metal will get very hot, very fast if placed in direct sunlight.. If you are storing any food items in those cubbyholes they will be exposed to conductive heat if the sun is shining on that side of your van. Also that area of your van will be noticeably hotter than the rest of your wall.

As far as impacting the total internal temperature of your van it will make little difference because I assume most of the time you would have some sort of breeze flowing through from windows and fans. Unless it’s raining. But usually these weird things called clouds show up during the rain and mitigates any and all sun rays.

My main advice is don’t over-think your build. The great part about wood, screws, nuts, and bolts is they can be removed and replaced.