Propane will stop vaporizing at -44ºF, so at that temp you won't get any gas flow even if it was in a open vessel. However, as it gets close to that temp the vapor pressure lowers toward atmospheric so an appliance that requires a large flow (i.e. furnace) can be starved enough that it won't be able to operate. The smallish regulators and gas plumbing in our RV's adds to the problem.
I've never actually tested it, but I'd suspect that somewhere about 10ºF below zero is about as cold as the furnace will operate.
Having said that, the volume of liquid in the vessel has a lot to do with how fast the vaporization occurs and exacerbates the flow problem at low temps.
Salem 29RKSS Pushing a 2500 Mega-Cab CTD!
Gotta go campin!