Oh, I was hoping you had a heating system that vents the exhaust out of the camper.
Heating by burners that release the exhaust into the air is indeed quite unhealthy and even dangerous. You should at least buy a CO-alarm. (they tend to be expensive, but there exist cheap Chinese CO-alarms, I hope they can be trusted ...)

The alternative strategy is to just use lots of clothes and sleeping bags. It isn't that comfortable, but what doesn't kill one only makes one stronger. I had a polish friend that was living in his boat, by anchor, and without working heater in December this year. I've also had nights in my boat with the heater off, quickly dressing the kids while they're still in their beds and before they complain it's cold, then getting out and towards the school...