nothing wrong with a portable gas heater ,we used one for quite a long time in our bedford when the fitted heater packed in . never a problem with the carbon monoxide we had a detector in the van one with a led readout and it never showed and it never went above a couple of ppm with the heater ,unlike when we boiled a kettle on the rings ,it used to go through the roof then . very little condensation either like many complain of . and you can get them for a few quid at boot sales , try the bridgend one on a sunday ,or the porthcawl one if its still there . ps there used to be some thursday and sundays at abergavenny ,thats another place where theres quite a few places to overnight ,bus station truck stop being one .