Ask your propane dealer if they add a de-icer such as methyl alcohol to their propane. This is used as anti-freeze as it combines with the water in the tank and lowers it's freezing temperature. Same idea as gas line anti-freeze. We add a small amount as a standard procedure for our outdoor tanks for emergency generators. They also have two stage regulators. You can do the same but of course the tank should be empty, transported with the valve close until ready to fill and only a small amount added. Here is a Texas government link that gives you some idea on the quantity. Hope it helps.
http://www.google.ca/url?sa=t&source...1e8Wcw&cad=rja