Hi Peter,
We do this trip each year from Calais down to Bretignolles sur Mer,(distance circa 430 miles depending on your route).
We travel down the A16/A28 to Rouen, then take the 'old' N138 to Le Mans, stopping over night at Beaumont Sur Sarthe (municipal site), this is 400 Km from Calais. The time that you get off the ferry into Calais is the decider for us as you don't want to arrive at this site late on in the evening. We normally leave Calais around 10.00, arriving in Beaumont around 15.00. You can do it faster by keeping on the A28, but beware, it's expensive if you have a caravan!!
From Le Mans south, it all depends where your destination is, as to what road you take and whether or not you are in a hurry.
Go on to Google and use their maps, they do really good road maps and you can also factor in campsites when you get your route.
Try http://www.campingfrance.com/ for sites, this web site will help you, and remember their is more info on the web, than buying camping books!!!!!!!!
Bon journey mon amis.