Last Christmas I met a family in Arc 1600 who had their camper van parked in Bourg st Maurice. This is ideally suited - it is an all year campsite with power to the caravans. Bourg is quite low at 800m so it doesn’t get too cold and the campsite owner runs a minibus to and from the funicular station, which is otherwise about a 15-20 minute walk away. The campsite is right next to the very good municipal swimming pool and just opposite the super U.
Bourg is 600miles from Calais, about a 9 hour drive in a car, though I suspect at least 50% more in a campervan.
John