Came accross these, basically a car style heater you connect to a heating source.
http://www.t7design.co.uk/index.php?main_page=product_info&cPath=65&products_id=180
Now as a quirk, if you are having a fire going, whats to stop you having a heating coil in the fire and a pump to pump water around into the camper, HAY presto, heater.
Humm got me thinking.