You also might want to check out Casinocamper.org. Until last year we'd not stayed at the Casinos but plenty of Walmart's or Big Box stores on our travels as one off O/N's. Now we love Casinos way more than WM to O/N = classic example of we never missed what we'd never had.
Last September/October on a major road trip coverage, out of 43 nights on the road, we only stayed 2 nights at Glacier NP CG, 1 night at a Seattle CG, and 1 night at Waterton Lakes Alberta NP CG. All but maybe half a dozen of the remaining dry camping nights were spent at Casino carparks on the Oregon/California/Washington coasts, and some we were invited to stay for a couple or more nights by their security personnel.
Freecampsites.net is another one of our go to websites, which lists some neat locations to overnight or longer and some besides lakes, rivers, vistas/views. We also use Sanidumps.com and try always wherever we stay or dump/refill tanks, to buy our fuel there, spend some dollars, or if a donation box put in a fair value donation.