If you have a van, you might not need too many campsites.
Vosges is great, haul up and loads of little tracks back down. There is a number of FFC centres in the area and the riding around Lac Blanc, St Marie les Mines and Greubweiler is excellent.
[/url]
013[/url] by ianvincent[/url], on Flickr
The road between the top station of Lac Blanc and La besse has loads of little tracks where you can park up.
Jura, only been to the area around Bugy (another FFc centre). Really nice and quiet with loads of riding.
Haute Verdon, a lot further but beautiful, not too busy, and assured good weather. Allos and down the valley towards Castillane is good. There is a really nice site called the Pommiers in the village just down the valley from Allos, or you can park the van in one of the top carparks at Allos ski station (awesome little bike park)
[/url]
049[/url] by ianvincent[/url], on Flickr
Another area worth looking at is the Vercours around Villard de Lans. There are loads of trails (FFC centre)and its really close to Grenoble.
This might be useful
http://www.ffc.fr/a_VTT/a_SitesVTT/index.asp