ReserveAmerica and Recreation.gov for govt campsites. iOverlander is a great community driven mapping software for all things vanlife/rv, but has a yearly subscription. I believe they still have a free ad supported version, but when they first switched to that model the ads were cancer. I use it enough that the $70/yr (last re-up, may have changed) is worth it. Other sites exist that are similar. Also, just figuring out where blm land and natl forest land are and either talking to rangers for that district is a good place to start.