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.
Near a public highway: As long as your vehicle is not causing an obstruction, there is no law against sleeping in your car overnight. On private land: in general, you should seek permission from the landowner.
It mean that between the hours of 8 am to 6 pm you may not park your car in anyone spot for longer than two hours or you'll be ticketed or towed. These are usually high traffic heavy business areas they don't want people parking long term and monopolizing the parking during business hours.