one of the first common problems i encounter when this problem happens after changing an LP tank is to check the nozzle at the tank. Alot of times the 'outer' seal of the fill nozzle will get stuck inside of the 'tank nozzle' and keep the 'truck nozzle' from **** completely onto the tank. Most of the time the truck just won't start up because it is not opening the valves.
BUT....
IF the tank nozzle is **** completely on and you say you have a freezing line? at the carb? or between the evaporator/converter and the carb?
I would first check the coolant in the cooling system and make sure it is full. Low coolant will make a system freeze up in a heartbeat. Also another common issue is a clogged up evaporator/converter, coolant not flowing through it will cause it to freeze up.
Do you smell LP? check and make sure all the lines where they connect are tight. Even the slightest leak can cause it to freeze up where the leak is.

There can be other reasons a line can freeze up like that but we have to start somewhere don't we? :o)

And obviously what edward says is not to be taken lightly, if you are not familiar with LP systems get someone that is before you attempt to work on this. Your ignorance could be a dangerous & costly a oversight in the end if your not careful.