Wal-Mart is the MOST EXPENSIVE campground or overnight that I have ever stayed at. We always stop at a walmart on night one of our winter trek south. The DW always goes in to the service desk after she has shopped to load up the RV....it is never less than $150-200 bucks of groceries, supplies ect....and asks if it is ok to get a few hours of rest in the parking lot. We have never been told no.
That being said, there will always be those who deploy the slides, put out the carpet, awnings and grill. 2 years ago I asked a guy parked in the back lot how long he had been there. He said 2 days and was leaving the next day....he apparently was visiting family that lived about a mile away. That seems a bit much.