Yah you may ask. Some places have strict rules on overnight parking and others won't really mind. Also depending on which one you go to there may also be a "cop" there circling the building so just watch out and ask some people before you get towed or something like that
I think you would be ahead if you would ask before you park. Each Walmart may have different rules.
For the most part in a car or truck they won't bother for one night.
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