My wife and I have stayed at many Wal Marts on our travels around the
West. We always ask the manager if sleeping overnight in our van is OK.
On those rare occasions it was not allowed, a local law prohibited
sleeping in vehicles when the parking lot was not owned by Wal Mart.
Usually we find this type of law in places that get lots of
tourists--such as beach cities. In those situations there are usually
signs to let you know.  Sometimes we have called the store in advance to
see if an overnight stay is allowed.
We think that it is very nice of Wal Mart to allow the stays.  We always
park in the areas they suggest and leave our area at least as clean as
we found it.
Lee Kouns