Some possible explanations for the odd and sometimes confusing possible causes of death in news reports:
One thing that can happen is the oxygen is "displaced" by other gases. Carbon Dioxide (CO2) is not considered as deadly a gas as others, but... a person can be affected by CO2 not really because there is so much of that gas, but because the oxygen was removed or "pushed out" by it. A lit stove can do this - the flame will use up oxygen in the area. This can also cause the lit stove's flame (especially if it is burning low) to go out, but the gas is still on. So now you have a build of CO2 and begin to build up propane or natural gas.
Carbon Monoxide (CO) is different, and is very deadly on it's own. The most common cause of CO is from a flame. A camper that has vents and windows open for fresh air is not at great risk. This is why campers and houses have gas stoves, with proper venting they are fine to use. The problem is especially during cold weather when people close up campers tight for heat. CO can build up to deadly amounts.
So these were simplified possible explanations.
Make sure your alarms are in good working order, and VENT, VENT, VENT!