One of the bigger ones for Wal-Mart parking is:
walmartrving@yahoogroups.com
BUT.... although it is corporate policy to allow overnight RV parking, more and more Wal-Marts are not longer allowing it due to either those abusing the privilege, local ordinances, or mall owners regulations.
TIP: Even if there are signs up that say "No Overnight Parking", always ask anyway. Many times they are posted but not enforced by WalMart or the locals.