CO is close enough to the density of air to not matter (about 0.93 or thereabouts, off the top of my head ). Thermal stratification is pretty transient unless the space is confined and the heat source continues to pump energy into the stratified zone.

What is probably a much more significant transport mechanism in a confined space under a factory home is the partial pressure of CO and other combustion products is greater in the confined space and therefore CO will seek entry into the home (and outside) until the concentrations are nearly equal.

Except for structural fires, the main mechanism for CO to move is partial pressure differences. Sort of like entropy... heat flows to cold, high things seek low places, high CO concentrations seek low CO concentrations...

HTH

Tom