This has worked for me...
Take a drinking cup of warmish-hot water and pour it down the side of the tank. You will hear the liquid propane inside the tank start to boil (turn to gas) a little bit. After a cold night using alot of propane I have switched back over to the "empty" tank, used the hot water on it and went through the rest of the day on the "empty" tank.
This has worked so well for me I have considered putting some type of very mild heat source in the tank compartments. Something like a light bulb mabey. This is just me thinking... so be careful now. This would help to squeeze all of the useable propane out of the tanks on those cold cold nights. And might just get us through till morning when we run low for whatever reason.
Hope that helps and doesnt get anyone into trouble.
Mike