I'm a full fuel tank guy. I don't leave it empty because how do you do this, where do you put 130 gallons of fuel? How do you plan the spur of the moment trips with an empty or near empty tank? Might work on smaller tanks, but not mine.
I don't buy the vapor pressure argument. Densities don't matter with gases, otherwise all of our atmospheric carbon dioxide would be at the surface because it is way heavier than the other gases that make up air.and water vapor, which is lighter than air, would be in the stratosphere. Doesn't happen. Gases are evenly distributed because there is enough spaces between molecules to allow all to mix, which is not true in condensed states of matter (liquids and solids). Mixing is the key. Heavier gases may sink for a while, but any disturbances , or enough time due to Browning motion, cause them to mix. That is why we can smell gas in the engine compartment even though we are standing a few feet above the bilge. The heating and cooling expansion and contraction cause all to mix. Best not to have a whole bunch of head space in my opinion. This will limit the oxygen that causes corrosion, or water vapor that might condense and cause issues.
