Here is an industry "Code Of Conduct" for parking in private business lots:
Here is another:
Our overriding principle is that we appear to be parking, not camping.
If we are successful, a passer-by should not see any visible difference that we are not simply parking while shopping. By doing so, we give no benefit to those who oppose free camping at our hosts, or others who seek evidence for the purpose of creating ordinances outlawing overnight parking. And by doing so we show courtesy and appreciation to those who allow us as guests in their community.
1. If you arrive before store closing, ask permission to park overnight.
2. Please limit your stay to one night. Arriving later in the day, and leaving early in the morning would also help reduce congestion in their parking areas.
3. Please empty trash in trash receptacles only.
4. Do not empty gray water or black water on the ground or in the sewer drains.
5. Please don't extend awnings, set up chairs and tables, or otherwise make our parking lot appear to be a campground. It can result in a loss of privileges. If you MUST put down your leveling jacks, be sure to place a large flat board under them to avoid damaging the asphalt parking lot.
6. Park so that your generator, if used, doesn't bother other rvers or their customers.
7. Park in areas suggested by their staff, or near other RV's. These areas are selected for your safety, and for the least inconvenience to their customers and their staff. At WalMart, that is usually toward the garden section. Flying J asks that you park in the parking lot away from the trucks.
8. Please express appreciation to the facility manager for the privileges they extend to us. That can include letting our hosts know that we like to shop there because of the courtesies that they extend to us.
9. Above all, be courteous and considerate to fellow rvers and our corporate hosts alike. If you feel an RVer is violating our standards, do not confront. Simply invite them to join our groupif you feel comfortable doing so.
10. The use of slide outs is discouraged. If you MUST extend, please park in a way that will cause the least inconvenience to those around you. In addition, some RVs require that levelers be used before a slide is extended. Thus the request that some sort of platform be used to prevent damage to the parking surface.
11. If you have a need to stray from the guidelines, the overriding principle should be to avoid or minimize any disturbance of others or to provide evidence that we are camping instead of parking. If it looks like you are camping, you are!
Both of these are from the WalmartRVing Yahoo group.