Marianne Edwards has written two books (so far) and many articles about traveling frugally and boondocking in the Southwest US. Her books, which are available here in eBook format, detail many interesting areas to visit on the cheap, and specific free camp sites. Each covers a particular area; one covers south Texas and the other covers Arizona. She is currently working on a book covering Utah, and plans to cover other areas as well.
Marianne and her traveling partner Randy are Canadians who regularly take several months off work, in order to live on the road in the southwestern United States. They are not rich people; Marianne is a waitress and Randy is a handyman. So how do they do it? That is what the two books and many free articles describe, besides the many reviews of free camp sites and attractions, which are a resource well worth the small price for the books.
Update: I bought the Texas book for myself, and am very pleased with it. I forgot to mention too, that the purchase of either of these books includes a free bonus eBook about the basics of boondocking, aka dry camping. This book contains information on how to set up your RV for boondocking, and practical advice on living in an RV without hookups.
The books are in pdf format, and only take a few seconds to download. Whether or not you buy the books though, the site is still worth a visit for the entertaining and informative, free articles.
BTW, if you have ever thought it would be cool to be able to drive right out onto the beach and set up camp, or even drive for miles on the beach on an uninhabited or sparsely populated island until you find just the right spot to set up camp, and then stay as long as you like and fish everyday for your meals, then you need this Texas book. I've done something similar in the Outer Banks of North Carolina, but the Texas gulf coast is a lot freer than the Outer Banks, and most people don't even know about it. In some places it's almost like being in Baja, but without having to cross an international border.
I haven't read the Arizona book yet, but it should be wonderful too. Arizona is the most well-known state of them all for boondocking (and gold prospecting, too); Quartzsite alone is worthy of its own book.
Click here for access to the free articles or to download the books.