Looking for free overnight spot on I-81 Virginia

I would call the Walmart in the area to confirm. We've overnighted in a few Walmart's and some had a "no overnight parking" sign posted but there were several semi's and rv's already there. I personally think that they have the sign posted just in case they have to say "we told you so" when someone acts like a fool. The book "the next exit" has helped us, you may want to check it out.