Do they allow overnight parking?

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.