There is also this site; Most free campsites don't have shower facilities, but every other day free saves a lot of money and adds to the adventure. https://freecampsites.net/#!(36.28819, -104.2895)A lot of the small towns in the Midwest have free campgrounds. They expect you to buy some groceries and/or gas and I was always willing to do so.
When you get ready to go on your trip let me know and I can steer you to some nice camping places between here and North Dakota and some of them will be free. Hell, for that matter you can camp in my back yard if that works. The Omaha area looks to be on your way. You have to come driving up in an Elio, of course ;-). Larry