writes:

Gasoline tanks are vented to atmosphere when they are filled - so they
purge themselves.

Obviously not, if they are open to the atmosphere.

Exactly. As the fuel goes in, the air is displaced, "purging" the
tank. A propane tank is NOT open to the atmosphere, so any air in the
tank stays there.

No, this is not purging. Even a "full" gasoline tank has some headspace
containing a mix of fuel vapor and air. A near-empty automotive fuel tank
will contain a large headspace with a potentially explosive fuel-air mix.