The policy of Walmart welcomes RVers to spend a night in their parking lot, as long as you have the individual location's permission. In some places, city zoning laws or ordinances will prohibit overnight stays in parking lots. Typically, larger cities are most likely to have these types of restrictions in place.
This number has been declining as zoning laws and city ordinances are increasingly banning overnight parking. Store managers may also not allow it. According to Walmart's website, “Permission to park is extended by individual store managers, based on availability of parking space and local laws.
Although there is no federal law against sleeping in a vehicle, staying in your car overnight is illegal in some states and cities. Laws about sleeping legally in your car vary across states, counties, and cities.
It is generally safe to sleep in your car overnight, providing the engine is turned off and you're parked in a secure location. Cars are not airtight, meaning oxygen can enter the vehicle during the night to allow comfortable breathing, even when the windows and doors are closed.
You can park overnight at Walmart, Sam's Club, Costco, Home Depot, Camping World, and a few other big box stores. You can also park along most city streets as long as you don't see any “No Parking” signs. Rest areas are a good choice too. Most stores don't mind overnight parking in their lots.
In cold weather, blankets or a sleeping bag may provide enough warmth. In hot weather, you may need to crack open your windows, but it is only safe to do so if you are not idling the vehicle. If it is too hot or cold for these methods to work, you will have to run your HVAC system in order to sleep.