These have been posted before, but perhaps you don't know about them. These are my favorite low cost camping places:
The following are 50% off with an America the Beautiful old age pass:
National forest campgrounds: http://www.forestcamping.com/dow/list/nflist.htm
Corps of Engineers campgrounds: http://www.recreation.gov/campgroundDir ... cyID=70902
National Park campgrounds: http://www.nps.gov/findapark/index.htm
Bureau of Land Management: http://www.blm.gov/wo/st/en/prog/Recreation.html and
http://camping.about.com/cs/campgrounds/a/public05.htm
http://www.rv-camping.org/PublicLands.html
Other websites with info about free or reduced camping at city and county parks or other sites:
http://freecampsites.net
http://www.uscampgrounds.info
RV friendly Casino camping: http://www.casinocamper.com/rv-parking-locations
My favorite AP for finding these campgrounds is Allstays for RV.