To further muddy the discussion of CO movement, remember that the two sources of CO inside an RV are the cooktop and furnace (if the heat exchanger is cracked). The CO from these will be hot, so although the specific gravity of it is very close to that of air, it will tend to rise with the rest of the products of combustion (mostly CO2, water vapor and heated air) until all those hot gases mix with the rest of the air in the RV. So it won't really go just to level dictated by its specific gravity.
Propane, on the other hand, isn't heated, so you'd expect it to seek low levels.
Also, remember that the primary value of a propane detector is that odor doesn't wake up humans. You can detect a propane leak long before the concentration is a problem when you are awake, but not when you are asleep.
2011 Sunseeker 3100
1997 Pathfinder Toad