Hello Ian,
such devices do exsist but as Shiba has pointed out you loose some heat to the main caravan, and I am not sure how effective they work in the awning.
Being canvas or nylon and relatively thin, any hot air put into the awning will very quickly disperse. You might have better success with an elecric fan heater, or a radient heater which at least you can feel the heat before is percolulates through the roof to the great outdoors.
There are several models of radient heaters, some use gas others use mains electricity, and probably comparable to the cost of the Truma kit.