What's the best way to kill mold in a cooler?

Use bleach — mix a tablespoon per quart of water, scrub, let sit for 10 minutes, then rinse thoroughly until the smell is gone. Let it air dry with the lid open for a full day. Vinegar and baking soda work for light mildew, but bleach is the only thing that reliably kills black mold spores. Don’t mix bleach with vinegar (toxic chlorine gas).