Seriously consider a heat source. While it is possible to stay in a car, van or camper shell with no heat at a ski area full time, the condensation drips from the ceiling inside the vehicle and when it gets cold the water vapor freezes on all the surfaces inside.
x10 staying away from open combustion heating in a small space, unless it is a PlatCat vented design. The amount of water vapor generated will end up as condensation everywhere.
If she is handy, have her located a propane heater from an old travel trailer or RV salvage yard and install it. These were sealed units that drew air from outside and vented the exhaust outside. No Fans, no electricity required from batteries to operate. A completely passive system with a pilot light. We had one in a 1980 21' terry travel trailer that we used all winter at a ski area. Dealing with propane bottles is fine if she is traveling around a lot. If staying in one place a larger 100 gallon tank is better.
She'll quickly find that adding insulation and a vapor barrier inside her cab over will make it warmer, dryer and save her money on propane. Putting heat shrink plastic film over the windows also helps alot.
If she is traveling around more and happens to have a diesel truck, then the Espar or Webasto diesel forced air heater work really well and consume much less electricity from a 12V battery than the current forced air propane units. Plus it will be more reliable than propane in really cold, below -15 temperatures. More money but worth it. She can install it herself and save some money, while doing this voids a new new unit warranty, if a used unit or a clone, she would be her own warranty station anyway. Plenty of You-tube and Installation PDF's online.
If she doesn't have one already a high loft (7"-9") winter down sleeping bag as a quilt or a backup if things stop working is important.
