We subscribe to the Overnightrvparking.com website, which has over 5500 free overnight locations listed, with most of them verified by other RVs who have actually been there. It's $17.95/year but the searchable data base, directions, and sheer size & quality of the database make it worthwhile to me. And if you file online reports, either new sites or verifying existing ones, you get a free two week extension to your membership for each one.
The database includes Walmarts and other potential sites that prohibit parking as well as those that do.