I'm guessing your referring to one like this:-
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This one that was fitted to a 1978 Bailey caravan featured in a Practical Caravan Magazine article.
This was released just before the law about open flued heating appliance in touring caravans changed, and subsequently this type of open flued heater was outlawed.
The reason being, because the flame burnt in the atmosphere inside the living space, it uses up the Oxygen from that space. That is OK provided all the permanent ventilation and the flue in the caravan is functioning correctly. But if the ventilation or the flue is obstructed, the burning flame will deplete the Oxygen to dangerously low levels. And its quality of combustion will deteriorate and it will rapidly produce disproportionately more carbon monoxide (the silent killer)
Whilst the heater may well be concurrent with the caravan, it would inadvisable to use it. It will almost certainly be due a service, which I doubt cold be carried through lack of spare parts.
As a gas engineer I would almost certainly have to condemn it, and disconnect and seal it so it can't be used. If you want to retain the authenticity of the caravan, by all means keep the carcase for the visual appearance, but if you need heating, either look to a new heater that hidden from view or even an electric panel heater.