LPG is heavier than air and sinks to the bottom. It also, from posts above, seems to be able to find its way through very small gaps. I doubt most floors are gas-tight and therefore any spilled gas will find its way int to the bilge. Eventually, if the rate of gas going into the bilge is greater than the rate at which the gas degrades, if it does, the bilge will be full of gas. There is no outlet for the gas it being at the lowest level and once full the bilge-gas will overflow the floor. At this point any floor level ignition source will ignite it. Sop what is the point of low level ventilation as it cannot disperse any gas. Low level ventilation is usually the bottom of front and/or rear doors which are way higher than the bilge or floor.
Or have I missed something?
Regards
Ditchdabbler
p.s. I understand that carbon monoxide is lighter than air and therefore low level ventilation will help disperse it but we are discussing heavier than air LPG