If you are 62 or older, all the national parks, COE campgrounds, BLM campsites honor the Interagency senior pass for free admission and discounted usage fees, usually 50% off camping fees. The cost is $80, and it is a lifetime pass.
Most federal campgrounds have 14 day limits on your stay at any one campground, btw. Exceptions do exist, however; eg. BLM near Quartzsite offer $180 for up to a 7 month stay.
In the East, I, too, would suggest freecampsites.net. Also, Allstays.com, which also offers an inexpensive app for smartphones that has filters for various points of interest that I've found particularly useful while on the road.