I assume you will have a car. If so, look into staying in one of the smaller towns in Provence. I definitely feel Arles is better than Aix (both nicer and more central), but with a car, you will probably want a smaller place with less difficult driving.
Without a car I'm not sure how you will do in the area around Sarlat for 6 nights; however, if you will not have a car, you should definitely stay in Arles or Avignon, for the transit links.
Are you seeing places around Carcassonne, or only the city itself? If just Carcassonne, one night is sufficient.