LPG won't last long if he runs his gas fire 24/7 when the vehicle is parked on his drive unoccupied for weeks or months on end. Yes gas is cheaper than running an electric heater, but he'd probably spend more in diesel driving to fill up with LPG every few weeks. If heating is required when the vehicle isn't in use I'd stick a small fan heater on a timer switch to run for half an hour per day. However, if he wants to prevent mildew and damp then ventilation is much more important than heat. I'd open all lockers, prop up seat cushions and open roof vents. Perhaps, instead of a heater, stick a desk fan in there on a timer to get the air circulating once per day. Far better to have a cold, dry van than a warm, moist one.