You can use mappy.com or viamichelin.com to work out your route, and fuel and toll costs. To find overnight sites you can use http://users.telenet.be/leo.huybrechts/camp1.htm which lists ALL campsites within 15 kilometres of the major routes through France, listed from North to South along those routes, so makes site-finding easy. Sites which are open all year are listed in red - and POI information is given too.
Not many sites are open in France in January - so make sure the site you want is actually open at that time - don't rely on information in books or brochures.