This is a useful link to all campsites within 15 kilometres of the major routes through Europe - easy to use, and with all the details you need of opening dates, etc. http://users.telenet.be/leo.huybrechts/camp1.htm
You can use mappy.com, viamichelin.com for route planning - and if you wish select 'without tolls' for instance - or use them to calculate just what your tolls will cost. We've been as far as Tarragona in Spain, and have done two routes - the first down the A75 from Clermont Ferrand south, Beziers, Narbonne, Perpignan, and then around the western end of the Med and into Spain at Le Jonquera - but we've also done the route through the mountains, via Ax les Thermes, and Andorra - towing - and that was a lovely route with a fantastic Spanish service station right at the top with views backwards into France and southwards into Spain.
You may find that in mid-March campsites which are open may be few and far between - but most French villages and towns have an 'Aire de Camping Car' where you can stay overnight, with hook-ups available on meters, water, and waste emptying facilities. Usually they are well signposted, and often in the middle of the town, so very convenient for restaurants, etc. Similarly, all along the coast you'll find motorhomes camping 'wild' and this seems to be tolerated in France much more than here.