At 6 feet deep, in most locations, it doesn’t swing much in temperature from summer to winter. Day to night, not at all. That was the point.
Water is denser that gasoline and collects at the bottom. Draw-off sumps are used to pull that out from time to time. Pump fills are placed off the very bottom.
Very little air can enter the tank as gasoline has a vapor pressure higher than the ambient. Without the exchange of air, moisture via condensation really doesn’t have a pathway in.
Thanks for the correction. I’m running off memory that’s older than most posters!