Check out http://www.allstays.com They have lots of information including some very detailed spreadsheets (also books) on campgounds and other faccilities some of which, like Walmart, Lowes, Cracker Barrel, Pilot-FlyingJ that allow free overnights (not camping, just park for the night).
I've found the owner is very active keeping the data up to date.
I have all the pertinent Excel spreadsheets on the RV support computer.
If you enjoy camping on BLM's (Bureau of Land Management), many of these areas offer free off-the-grid camping. Here are a couple of web sites that may be helpful when looking into BLM camping: