A while back I read a very thorough article on cold weather issues with propane. Can't find it now, but a similar one (by www.marshallexcelsior.com
) mentioned that water in the propane is a major culprit of freeze ups. As propane is used, water vapor comes out of solution and can condense into water then freeze at temps lower than 32F. Combine this with the fact that as a gas (propane) expands, it gets colder. So the tank, lines and especially the regulator can be colder than ambient air temps, especially when gas is used rapidly like in a high BTU furnace. So not only do you have to heat the tank, but the pipes and regulator as well.
The water typically comes from the wholesale/retail propane distribution system, unless you leave the valve open on an empty tank for a while.