|You can look up National Park CG's Here, http://www.nps.gov/findapark/index.htm
or National Park CG's here, http://www.fs.fed.us/
you can also look up both here, http://www.recreation.gov/
you can also look them up & get reviews here, http://www.rvparkreviews.com/
That may not have been the answer that you were looking for, but to my knowledge, there is no one single place to find them all.
you can also click on the Campgrounds tab at the top.