Covered storage for my van during hot summer or uncovered?

Put a skylight in the covered parking over the panel(s), as all you need for power is likely to be to keep the batteries full, and maybe a fan running?

I think I would find a way to either do the above, run a cord, or get a small portable solar panel before I would leave it out in the hot sun all the time. Everything degrades in the sun, for the most part.