Suggest you rent from a company close to a channel port. You'll be be able to to rent a 4 berth camper for less than a large car from Avis.
You'll be able to get foot passenger returns for 10 pounds each via Newhaven- Dieppe, (PM me to find out how this is possible)and a quick train journey to Rouen will get you to the camper and on your way. Picking the thing up at Le Mans would be ridiculous, as you'd have to pay return fares UK - Le Mans return for each of you.
For instance these peeople
http://www.masters-rouen.com/ will give you a decent size ( 7 person (theoretically !) camper for 700 pounds a week with ample free milage, and if it's not big enough you can always buy a cheap tent from Carrefour, Auchan etc
Good luck