With a lot of help from Paul Smith and Ivo, here's a map showing the locations of the pbp start and the Parc-Étang campground:
Note the "RER" near the middle of the map--that's the train station. RER Line "C".
- GoogleEarth KML File
Map to the Start—Gymnase des Droits de l'Homme (Human Rights Gymnasium)
I think the other campground mentioned on randon (huttopia) is about the same distance from the start but in the opposite direction (i.e. east of the start) in the big wooded area south of versailles.