I fully agree with Mobile Matty,
A portable gas heater does not have a flue pipe, instead it puts all its flue products into the space it is heating. That is why they should not be used in small unventilated spaces. An awning is usually quite well ventilated, but because the surfaces are cooled very well by the outside air, the steam and water vapour in the flue products comes into contact with the chilled surface and that cause s the water vapour to condense onto the fabric, so it is not so easily ventilated away.
Just to put it into perspective, as a fairly rough guide, for every kg of gas you burn you will get about 1kg of water vapour.
The problem does not occur when you use the heater inside the caravan, because all the flue gasses are ducted away through the chinney.