Another advantage of Walmart camping is that every Walmart looks the same. That means when you stop there to camp you can tell the kids you're in Orlando Florida when you're really in Tupelo Mississippi. Heck, I've camped at our local wally world and told them we were in California. I even had the wife convinced this lady that was unloading her groceries was Oprah Winfrey.
If you camp overnight, may as well stay a week and soak up the free hospitality while fantasizing about your "exotic" Walmart location.