Wal Mart is usually very friendly to people in RVs using the lots to sleep overnight. It's a pretty well known trick in the RV community. Usually they're older, retired folk using the lots, and they spend money in the store. The company has a pretty chill attitude towards it.
That's an entirely different thing, though. Those people also probably buy their dinner and traveling supplies there, too. They're not just creepy degenerate scumbags shitting up the place, literally and figuratively.