You need the 'Route du Soleil', straight down in a line rfrom calais to Barcelona, through Troyes, Lyon, Perpignan, towards Marseille, then turn right, follow the coast road through to Barcelona! There are various toll roads, all good. It costs around £30 North to South. Far better than the 'B' roads, and quicker too. This takes you around Paris, and skirts along the side of the Massif Central.
Don't forget at that time of year you will get limited daylight hours so the make reasonable progress your best bet might be to use the Autoroutes. Having said that we have experimented with travelling on the N roads, usually coming north as we leave the Med. A couple of good stretches are from Troyes to Challons and from Reims to St Quentin but these two together only account for about 130 miles of what is a 700 miles journey. Treat yourself to the Michelin Road Atlas of France, this is an excellent guide and you can see the sections quite clearly.
If towing a caravan I think around £60 each way, that's just down to Marseille. Caravans/trailers are charged extra if above a certain height, I think +2m. Under this it's the same rate as solo car. But if you have plenty of time why go by motorway. There are good RNs that run parallel to most of thr north south autoroutes. See the country and towns and save a packet. Also avoid the route around Paris like the plague. Go south from calais past Reims to Dijon, only about 20miles extra but no traffic jams.
Sid
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We found the toll roads to the South of France to be expensive, I think about £50 each way with just the car. On the way back up France we tried the N roads which took us through some very nice scenery and were far more picturesque but took us a very very long time. My advice would be that if you are in a hurry to get where you are going or if you don't like winding through small towns and trying to remember to slow down through them so you are not caught speeding, definitely use the toll roads. You can just get on one and sit at 80mph for miles and miles. However, if you have plenty of time and don't mind only doing short distances each day, then use the N roads. I would definitely second David's suggestion of buying a really good road atlas. We bought a cheap one before we went this year and got lost a few times! The really good ones are a bit more expensive but I am definitely getting one before we go next year.
Get a Michelin road atlas---If youn goin from Calais--get down to Paris then go N7-then N9-to Clermond Ferrand--then A75 (free autoroute) to Pezenas-then N9 to Spanish border----post again if you want more details----
A day of all things motorhome and campervan. The show is held both in the main indoor arena and across the grounds. Catering, hot drinks and food options are available. Easy access to the site and free parking included. Win one of five pairs of day tickets