Many resorts have campsites open in winter (Caravaneige). Try Morzine (Les Portes Soleils) with two sites, one at the end of Morzine, the other in the next viilage down the valley. You can also camp for free in the car park (near the Gendarmerie and supermarket) in the centre of Morzine.

Make sure your vehicle has maximum antifreeze and is well insulated. Take an extra electric heater for campsite use. Two years ago, just before Xmas, the temperature dropped to -28C and stayed below -8C for over a week. Make sure tyres are in good nick, and take snow chains (you may not be allowed on some roads without them).
Patricia and Guy Featherstone