Air has a density of 1.0, by convention. Carbon monoxide has a density of 0.97, making it slightly LIGHTER than air. If it's being generated by faulty heating system, it's going to be mixed into all levels of the space, so having it on the ceiling of the relatively small RV bedroom is probably as good as it gets.
The only way to test a CO detector (besides the battery test) is with CO. Blowing on it loads it with CO2, which is not good for the sensor. Neither is using aerosols like hair spray, air freshener, or cleaners near it.