Geoff,
You are the same as me, electricity is not an option, I have a not really reliable eberspacher heater in the van and after the heartache of last wineter I gave up on it in the end so I tried all sorts of gas systems none of them really safe or reliable because some had flames etc etc.
In the end I went to HSS and hired a basic gas office/shed type heater the ones with the tank in the back, I wont bore you with the whole story but yes you need to leave a window slightly open or vent it some how.
They have an automatic cutout on them so that if there is too much carbon monoxide they cut out. Imagine the scenario, workman in a hut on security on a freezing cold night puts heater on high with the doors and windows shut. Imagine the rest yourselves.
I had mine on low overnight with a duvet over the TM with one side open so the heat got in, yes there was slight condensation in the morning but nice warm van and no burst pipes or ruined chems.
Murky