We've done several routes down to Beziers area (actually Grau d'Agde). Our favoured one now is Dunkerque, Hesdin, Abbeville, Rouen, Chartres, Orleans then down the N154 to Salbris where we stay overnight. Then it's on to Moulins, Gannat,and then down the A75 from Clermont Ferrand. We try to avoid tolls because these add up and because we're cheapskates!!!!
I'm not sure what route Tom Tom would give you but Viamichelin shows a route heading towards Paris, skirting Paris and then going over towards Orleans on the route described above, but sticking to the A71 autoroute with tolls of 85 euros in total, with a caravan.
There is no 'best route' - but we've found one that we're happy with, and we can get down to the Mediterranean, spending only around 14 euros in tolls by keeping off the toll autoroutes exept where we consider it sensible, to avoid a city with hold-ups, such as Orleans, for instance.