erm 3 pieces of advice 1. take your time and use the D roads, as the autoroutes are expensive and whilst they are usually traffic free and great for covering distance you see more of france going on the D roads
2 always keep a full-ish tank, the whole country grinds to a halt on sundays so finding fuel may be tricky, your card may or may not work at the unattended garages!
3 France is beautiful but my particular recommendations are to visit Fontainbleau (big forest full of boulders for climbing - kids love it so do climbers!), theres a great campsite at Milly-La-foret, and around Dijon is fantastic too, My wife and I spent 4 weeks travelling round living in the 110 going climbing, if you take a hammock and have a roof rack you only need to find 1 tree for instant accommodation!
You may find that the local Carrefour hypermarket sells all sorts of useful things for the landy too...
Enjoy it
Mike