Ok so its not all free. But I am surprised that there is any free camping at all. I have written about our stay at a fantastic site in Maine and the one in Ohio. We also found one in Delaware a couple of years ago.
So how do I find free places to camp? My first source is the free camping website. You can search for sites in area you want o visit and reads reviews from other campers. There is an app for Android but all it does is take you to the website.
Another place to check out are Walmart parking lots. There are a couple of Apps to help you know which ones will allow it: Overnight parking and Allstay. Allstay will cost for the download but it also gives you many other pay camp sites as well.