Didn't I read somewhere there's a way to sanitize water that involves a plastic bottle and a lot of sunshine? Seems to me, water might be the one consumable that'd benefit from being left in direct sunlight somewhere in the car. Not saying I would, just...

On the distilled vs. bottled: If the bottled water really is just tap water, then it's probably chlorinated. One of the water storage FAQs I found said that you didn't need to treat tap water for storage of < 6 months or so because of the chlorination. Distilled water is probably not chlorinated, but as mentioned earlier also may not be filtered / treated either, so anything that's in there will be free to grow, I guess. Bottled water probably wins on a cost / convenience comparison to distilled.