Hi wld - keeping warm in a campervan can be a pain. Our motorhome has central heating which runs on either 240 volts mains or butane/propane gas.

If your camper doesn't have built in heating you could always use a fan heater but that assumes that your van has a mains input and the site you are staying on has electric hookups. Bear in mind that hookups on some sites have a relatively low rating (say 6 amps) and if you plug a fan heater in - or a hairdryer or a combination of lower wattage appliances you're likely to trip the mains out and you'll stay cold. Most of the better sites have 16 amp hookups.
Ray