There is no national policy -- Polite to ask
The polite thing is always to ask permission when you plan to "squat" over-night on someone's private property. Wal-Mart has no official policy but there is a reasonable expectation that you can spend the night in the lot in most of the store locations. Rules of thumb: 1). Park in the outer areas, so as not to take parking spaces from the regular shoppers. 2). Remove & dispose of all trash in the area of your parking area -- whether you put it there or no. 3). We make a point to ALWAYS ask permission of either the store manager or the local Wal-Mart security. 4) And most important -- ALWAYS purchase something from the store as an indication of courtesy. In certain areas of the country, security will be less welcoming to Pick-up/campers than to traditional RVs.
Our personal choice for free over-night parking -- is off-road in National Forest and BLM lands or commercial truck stops. Nearly all truck stops have areas reserved for RVs, campers and passenger cars. Do not park in any area reserved for commercial 18-wheelers!
Thanks for the info -- Updates here
Thanks for looking up that information. The list available on the URL stated above is current only to August 2002. Chuck Woodbury's site http://www.freecampgrounds.com/noparking.html has an updated version current through April, 2003. It costs $3.