DO YOU ALLOW RV'S TO PARK THEIR OVER NIGHT WHEN PASSING THROUGH?

I don't really know about their parking policy, but overnight parking you should be fine if your just relaxing in your RV overnight

While that used to be a common practice, many Walmart stores have moved away from that practice. Your best bet is to call the Walmart you wish to overnight at and ask the manager. If you cant call, go in and ask the store manager for permission.