dave - -

propane won't freeze this side of cryogenic temperatures.  But getting water
vapor into a propane system is easy to do.  It usually gets in there as
vapor along with the propane when your bottle gets filled in the summertime
- when nobody expects it to freeze.  

Take your system to the local professional propane dealer - - -

That's right - - the propane dealer - - not the supermarket or filling
station that fills propane bottles or exchanges them.  You'll probably have
to look him up in the yellow pages.  Be prepared to drive down into the
local industrial district during normal working hours.   The dealer will
purge your system and add a small amount of "antifreeze" - usually methyl
alcohol to your bottle.  This will tie up water vapor that gets in the tank
- - at least for the next few months.    While he's doing this, also have
him leak check your system. 

Some really good RV dealers can also provide this service.