The following advice has been gleaned from various sources.
The main thing to bear in mind is that if it is cold enough for good snow it is cold enough to freeze your water systems this can be a real problem. Also remember the lower you are the less chance there is of freezing up. So staying at Bourg St Maurice at 800m you are less likely to experience really cold temperatures than staying at for example Les Menuires at 1800m.
Some vans will be offered as winterized but "Winterization" varies in efficiency! It should be noted that lagged outside tanks will freeze in persistent temperatures of below 0 deg C. The best winterization provides a double skinned floor which contains both the fresh and grey water and is heated and the outlet grey water valve is within this compartment i.e. mainly German manufacturers such as Hymer. Good winterization also means thicker insulation on the walls – check your handbook.
Central heating is essential - without it you will be cold and damp and miserable. Keep it on at night though lower the thermostat to save on fuel. If you are on hookup a small electric fan heater left on will keep the van warm. Turn off your water pump in case the frost discharge valve on the central heating is released during the night. If the frost valve does release, pull it up and use a peg to hold it in place until it has warmed again.
When using gas, Propane is essential. Do not use Butane which freezes at zero, 0 deg C.
Do not use cookers for comfort heating because of the carbon monoxide problem.
Make sure you have warm sleeping bags, not flimsy summer ones, or good warm Tog duvets, or both. Thick socks and extra clothes help. The head will now account for 90% of your heat loss. Don’t be ashamed to wear a hat in bed. Save on fuel by wearing two or three layers of clothes in the van - don't listen to teenagers who think it should be warm enough to enable just a thin tee shirt to be worn inside!
After skiing, clothes are often damp but will dry easily, but not in the wardrobe. Have plenty of hooks for gloves, hats, lift passes, sunglasses etc. Condensation and cold will be major problem without insulation. Firms that make such protection say that external screens are best i.e. Taylormade or Silverscreens.
Snow Chains are a legal requirement in the winter season over 1400m. You may be stopped and asked to show them even when the roads are clear. Practice putting your chains on in the UK. When it is dark, snowing and blowing is not the time to try them out! Carry a large mat or piece of plastic to lie / kneel on as the moment when you need chains it is often slushy. If you have a front wheel drive vehicle try and load weight forwards. Do not drive on chains if you don't have to - without snow they are bad for your tyres.
Make sure your continental warning triangle is easily accessible and you have a reflective jacket.
Fill up with diesel in the mountains just before resort, the diesel in the mountains is better diluted for cold temperatures.
Leave the hand brake off but remember to put it in gear. Use your engine going down hill to break when possible. A 1000m-drop can causes some very hot brakes. Make sure your washer water has additive in it – again buy it in resort, theirs is better.
A mat for outside on which to remove boots can be helpful and also have to hand a stiff brush to bang and brush snow from boots. The brush is also good for clearing window screens when snowing. Be proactive and keep your van and the area around it as clear as possible
Extend any vents on the roof to ensure they don't get covered in heavy snowfall. Most vents have screw on tops so adding an extra 30cm should be easy - and potentially life saving if the gas flu is kept clear! Most resorts have a swimming pool with showers that only cost a couple of Euro to use. Use these at the end of the days skiing.
This has 2 advantages.
You can even hire speedos as shorts are not normally allowed (buy them in UK before going it's cheaper).
This is Very useful when not on an official site. Take a portable generator. They cost £300+ and can be worth every penny. Again, indispensable if parked away from mains electricity. Ideally you will need 1000watts as the minimum if you wish to power a heater.
Unleaded is available in all resorts - Propane is not, and it's cheaper and easier to carry too. When you're in your van, power your heater from the geny and save the gas. You can also use the geny to assist you (or others) start your engine if you've been parked up for a week in -20.
Think about fitting a 2nd leisure battery. Cold kills batteries like nothing else. Having 2 gives you a whole world more power to keep lights etc running. Even if you have a solar panel, you may not get any sun for several days, and many valley bottoms don't get any sun at all Dec-Feb.
Air your van out for at least 10 mins every day. This may sound crazy and goes against every instinct when its below freezing, but condensation will build up rapidly in your van, especially if cooking daily. The air in the Alps is generally quite dry (though cold) and your van should remain pretty damp free providing you let the damp, moist air out daily.
Don't use an electric step, it could (and has) frozen in the open position. Alternatively get hold of a plastic bread or beer bottle crate and use this turned upside down. Makes a great step and you can stamp your feet on it to clear snow from your shoes/boots. The snow falls through the holes in the crate.