I guess I'm one of those "mind your own business" people. Really, it's Wall Mart's decision to address the issue, no matter how much it may offend the public. Best solution is to go talk to the store manager and see what it's all about if it REALLY bugs you.
"Hey Mr. Manager? I noticed an RV in your parking lot for at least the last 2 week. Just curious, do yo know why he's there? Do you know if he needs any assistance?" ... nothing offensive in that, nothing judgmental, nothing confrontational, and handled in a professional manner. (and nothing says YOU are offering assistance either.)