In July and August you travel alongside the whole of Europe. You book ahead, you queue at the tunnel, and the best spot by the water is taken before you arrive. From the first week of September all of that falls away. The French, Germans and Italians are back at work, the coast empties out and the mountain passes go quiet.
The weather is still good. The Mediterranean has been storing heat all summer and is often warmer in September than in June. In the mountains the days are clear and the nights are crisp, which is exactly the weather in which a van with proper heating becomes a real advantage.