Most Walmart Stores do allow overnight RV parking.
But not all Walmart locations or Walmart Stores allow overnight RV parking.
Some towns have local ordinances that make it so that Walmart cannot allow overnight RV parking in their parking lots.
So some Walmart stores may not allow RV parking overnight.
If the Walmart store does not allow RV parking overnight there will be a sign in the parking lot or near the parking lot.
If you do park in the Walmart Parking lot and you're not allowed too then a cop will knock on your door at night and just warn you to move.