South of France

It will take you 2 and a half days to get to Frejus and the same returning from the Perpignan area with a day to travel between the two.  These are the direct timings taking it steady, driving for about 6 hrs a day and without breaking your journey.  Having 4 weeks to do your trip is plenty and would give you a reasonable time in each area.  Going in Sep you will have no problem finding sites and I would just suggest you join ACSI to take advantage of their discounts.  Not sure I would want to take a week to get back as at the end of Sep the weather could be changeable as you get North.  So I would suggest you do your enroute stops on the way down as the weather should be good. I would be tempted to visit the Champagne area on the way down where the Vendange maybe taking place and then perhaps Gordes in inland Provence. Three nights in each area is needed to give you any chance to have a good look round.  The message is be very flexible; there is no need to book - we went to Frejus last Sep and it was surprisingly quiet.  Avignon and Aigues- Mortes  are both worth a visit. From Perpignan there are lots of things to see and do and these were discussed in another post  a couple of weeks ago.  A lot of sites in France close mid Sep but nevertheless there are plenty which do not; you just need to bear this in mind.

You don't say which ferry you are taking.  If Calais then we get an afternoon crossing and then stay near Calais  and hit the road early next morning.  The quickest way down is via Reims/Troyes staying well northeast of Paris and then the Autoroute (A6) this is sadly quite an expensive route toll wise. There is of course the non autoroute way but this would add at least a day to your trip and tempt your tolerance with the 80kph limit and proliferation of speed bumps! If you use the Caen crossing we get the afternoon crossing and stay at Riva Bella just outside port.  Then heading direction Lyon Returning from Perpignan you can go either by Toulouse or Clermont Ferrand both routes have quite a lot of toll free kilometres.

You will receive lots of suggestions as to places which are worth a visit. Do not try to cram too much in; maybe just do to Provence and back with stops enroute this year - you will find that a month will not be enough - and  then the west side another trip?  We used to live in Languedoc and have also visited France many, many times over 40 years and yet there are still areas we have not visited. You have to make your decision!  We are off to France at the end of Aug for 5 weeks but will (probably) stay on the west side.  Who knowns.

(Post Edit.  Just seen BBs post.  His timings are I think a little optimistic for towing and I would stick with mine which are based on doing the trip many times.  To echo BB - enjoy enjoy  enjoy!)

Gaz,  my advice would be not to try to cover the whole of the south coast of France in one trip. Go west of Avignon or go east of Avignon and concentrate on one area.

Although in early summer last year, we went in neither of those directions and spent all the time we had not far from Avignon itself - a lovely old city - and wandered around markets in St Remy de Provence, Menerbes, Gordes and Apt, nearly bought antiques in Isle sur la Sorgue, did a bit of bird watching in the Camargue and found remote beaches down there, did all the Roman history things in Nimes, Arles and the Pont du Gard, visited vineyards, bought wine, had lunch most days, took photos of lavender fields, and got very sandy in the ochre pits at Rousillon and Rustrel.

We like that area so much we just hit the motorways these days to get there soonest. 

BB.  No wish to get into a long discussion about timings etc.  Just trying to guide OP.  France is a big country. Distance Calais to Beziers is 650 miles.  I do 3 x 2 hr stints (not 2.5 x 6!!) and have, over many many trips, found that comfortably it is 2 and a bit days towing a caravan on the Rouen/A75 route.  We like to stop mid afternoon to visit the village/town nearby and have a relaxing meal in the evening.   If you want to do 3 x 3hr stints then yes 2 days is achievable. I even met one guy who would do it straight through towing Argeles to Calais!  I once did it with one stop but that was exhausting and not repeated.  Solo, I easily did the trip with one stop, or in emergency, straight through.  Just, as a caravanner, who knows the route well, trying to give the OP some indication to help his planning.  He will have a great trip whatever he does. 

Wherenext, yes in Dec we would often have temps in Dec of 18 deg + but that was usually linked to vey cold nights; I got sunburnt on my folically challenged head on 6 Jan our first year living near St Chinian!!!  Also, the coast just SE of Marseilles is lovely.