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Originally Posted by DickR

Presumably that advice about keeping tanks full to avoid condensation inside pertains to gasoline tanks and is not meant for propane tanks. That won't happen with a propane tank. The pressure inside follows the vapor pressure curve for propane, and the top 20% or more of the tank volume is occupied by propane gas, with propane liquid below that. As the tank warms during the day, the liquid propane in the tank absorbs heat from ambient air through the tank wall, and the internal pressure rises. As the tank cools at night or after a cold front passes, condensation will occur inside and pressure drops, although this occurs slowly. But this is propane vapor condensing back to liquid. At some low temperature, well below zero, tank pressure is insufficient to deliver propane vapor to the appliance.

When the appliance uses propane as fuel, the tank pressure drops a bit, some propane boils off to maintain pressure, thus cooling the liquid accordingly, followed by absorption of heat from ambient air through the tank wall.

Since the tank is essentially always at pressure substantially above atmospheric, there is never any introduction of water vapor from the air. If there were any moisture present in the propane, its partial pressure would be way below that needed to cause condensation to liquid water droplets at any realistic temperature. Any water vapor present above the liquid would be flushed out quickly as the appliance uses the fuel.

Good info. So how do they freeze. I did look it up and it seems it can happen.