Don't forget the mantra..."Carrefour will provide", you can buy most things that you might need in Carrefour and any camping gear you've forgotten in Decathlon.
The important ones are...
Passport (& copy)
V5, Driving licence (& copies)
Tickets (for travel/Campsite etc.)
Breakdown insurance, extended for France, check the terms of the cover IIRC most policies require you to have taken BOTH sets of keys with you before they will stump up for lost keys
Car insurance, extended for France ("Green Card" cover), also consider adding a driver so that each car in your group has a second driver in case of illness, tiredness on the way back etc.
Travel insurance and EIHC (these two are not the same thing!)
Plus....your brain with regard to keeping the things above safe! Don't leave your car keys on a table on a campsite and be surprised that your car has gone!!