Some Walmart's do not allow overnight RV parking in their parking lots and if they do not allow overnight RV parking they'll usually be a sign in the parking lot entrance letting you know about the overnight RV parking not being allowed.
If in doubt you can ask someone in the Walmart Store if you're allowed to park overnight and most Walmart Stores will allow you to park your RV or camper overnight and sleep as long as you move the RV or camper the next day.
It's best to also go inside the Walmart Store and buy some supplies or something before staying overnight but you're not required too.
The worse that will happen if RV parking overnight is not allowed is a police officer knocks on your door and asks you to leave.
I've had that happen to me at one Walmart before when I was tired and decided to sleep for a few hours and so I just left and never got a ticket or into any trouble because of it.
But most Walmart stores do allow overnight parking and some Home Depot stores do as well.
I asked the Home Depot in one state if I could sleep overnight in the RV on their parking lot and they said it was just fine and that they welcomed people overnight parking in their RV.
I did buy some stuff as well their but I wasn't require to do so.