It's a bit odd that they report it as CO2 poisoning but then report that a burner was left on and there was a strong odor of gas in the coach. The only scenario that I can think of to allow for both is if they left a burner ignited and so much oxygen in the camper was burned and soo much CO2 produced that the flame went out.
The next thing I think of is our trust that the detection devices in our homes will do what they should. I can test the smoke detector easily. I can even test the LP detector carefully. Is there a safe way to test the CO2 detector?