If you are not tied to a route, how about the west coast route we use every year (although we start from Cherbourg). Here's a selection of all year sites, many near autoroutes. All in caravan Club Europe volume 1
Heric, between Rennes and nantes
Futuriste, at jauny Clan near Poitiers
Moulin Averne at Pons between Saintes and Bordeaux
2 sites at mirambeau, just north of brodeaux
several sites near St Jean de Luz - just north of Spanish border
Mendigorria just south of Pamplona
Savinan just north of Calatayud and south of Zaragoza
Manzanera between Tereul and Valencia
Navajas also between Tereul and Valencia.
This way avoids the middle of France which can be very cold. If you look at a map, the line from St jean to Valencia is not far off straight and it's 4 lane virtually all the way.
If you try it and see a MB 270cdi / Burstner S500TS going the same way, give us a toot !