I think there is a difference between stopping at Walmart to get some sleep before another day of driving and stopping at Walmart for several nights to go sightseeing.
If your plan is to sightsee for several days, then find a campground/RV park.
If cost is an issue, then check out freecampsites.com, etc. There are a lot of legitimate options out there.
While you may get away with doing it, it WILL result in more communities and/or Walmarts banning overnighting in parking lots. Then you lose that option when you really need it.