Similar thing happened in WA years ago, man and his young daughter were free camping and had blocked the vents etc in their campervan to keep out the cold, I think it was the burnt gas from their gas fridge that overcame them, very sad.
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Vic
Hi Ace Pop Top Campervan & A'Van A'Lite Camper Trailer.....
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One would assume there will be an inquest or the Police will determine the cause. If as we suspect they were using a stove to warm the van, even if it remained burning it well may have produced enough carbon monoxide to be fatal.