Ditto on Militarycampgrounds.us It is a great resource and kept up to date. Most military campgrounds have weekly and monthly rates, but also have rules against "homesteading" meaning you generally cannot stay there forever.
Any National Park can sell you an Interagency Senior Pass (used to be Golden Age Passport) for a one-time fee of $10 and it will let you into National Parks for free and discounted camping fees, some as much as 50%.
You may want to consider volunteering or camp hosting at state or national parks to hold your costs down and to be able to stay for a longer period of time.
Military retirees have the benefit of military campgrounds and RV parks that our civilian brothers and sister don't enjoy. For example the ONLY campgrounds on Key West are part of NAS Key West and you can dry camp for $12/night. The nearest commercial campground is on Stock Island and is over $100/night. America's way of saying, "Thank you for your service!"
http://michigantraveler.org/2012/03/31/key-west-fl/