When I drove for a living we would park up for the night at Walmart. I always felt safe there. Park away from the building so your not in the way.
From what I have researched, you may park overnight as long as you park far away from the store and don't set up equipment such as grills, lawn chairs, etc. One article suggests you let the store manager know that you will be there overnight.
Yes, as previously mentioned, park away from the store and you'll be good.
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