This web site lists all the camp sites in France and is also available in book form, published annually. The book is in French (of course) but has a good English key section.

On the web site you can select by criteria, one of which is "nudist site" under general information.
Some years the book has had a list of all the naturist sites in the front or back.
You can get the book in most French bookshops and many newsagents and probably from the French tourist office in the UK but I haven't checked whether it is available on mail order through the web.

Edited to add:
It is available through the web site but the 2010 edition is out of stock and the 2011 edition will not be published until February.

John,

many thanks for that. I dont plan on going until April and so Ican wait for the new Feb edition!

Hi JayOB,
You will need to either check on the web for naturist camp sites in France or as previously suggested the French tourist board. As for driving through the French country side until you are tired/bored, is easy enough as almost all decent sized French towns have a Municipal camp site which are usually clearly sign posted. While these sites are not Naturist oriented they are in our experience clean well run and cheap, as little as 13 Euro's a night. A great way to see France en route to your chosen Naturist destination.

A thought just occurrd to me:
When the book which I suggested does not have a separate reference list of the naturist camp sites I have used the web site to find them and a highlighter pen on their maps to mark the locations.

I did something similar last year. I went by motorbike from Dover to Calais, and headed down to Brittany. I had put a few naturist sites into my satnav before I went, and had the aim of getting to one called La Pinede, in West Brittany. I was expecting to stop en route, but in the end I made it in one day. The campsite was great, see my review elsewhere on this forum.
If I were to repeat the trip I would go from Portsmouth to one of the French ports further south than Calais. The crossings are dearer, but offset the cost increase against fuel, tolls and time spent driving and they are worth it, in my opinion.
If you want any more gen on the campsite you are welcome to pm me. Their web address is http://www.cnbs56.com/
I can speak a bit of French and it was handy on the site as not many seemed to speak much English; they were very forgiving of my French, and made me most welcome.
As is usual for these type of trips, go with the right mindset and you will have fun. Just don't get sunburnt like I did!

.. .. .. .. they were very forgiving of my French, and made me most welcome.

That is typical of the French and even more typical of Bretons.
If you attempt to speak their language they will go far more than half-way to help you.
But if you don't then they won't.