Here is the Official Walmart Web site where they list the Stores that DO NOT ALLOW overnight parking.
My experience is go in and ask the manager and buy something and they may do an exception.
Not sure who the publisher is, and where they get their information on no-park walmarts. I did an overnight at the Bleachery Street in Ashville NC. It's on the list. In the morning we saw 5 other RVs in the lot. Felt right at home.
I live in Asheville, and have noticed that all the Wal Marts in our area have "No overnight parking" signs posted on about every street light pole in the place. I think the store on Bleachery blvd. got some squatters that moved in and put up a mail box out front! My wife & I used to check out all the RV's there when we went shopping. Haven't seen any there in months though.