We should be doing everything within reason to be good stewards of Wal-Mart's generosity. I continue to see Motor homers parked in our Wal-Marts here in NC with their levelers down, and slide outs deployed. One this week even had his awnings out. Be a good steward of their property, and ask the manager before you set up for the night.
If you park in a public vehicle area over night, be willing to accept the risks associated with your decision.