To to OP:
I use 20 lb (gas grill) tanks for external tanks on my MH. During the past month the temperatures here have been down to about 25F at times. My propane furnace doesn't have a problem with low temperatures.
What it does have a problem with is when there is only bout a half gallon or less of propane in the external tank. The furnace does not want to ignite when the fuel level is too low for it to operate.
Another thing that I have noticed is when the propane level gets too low that there is a change to the quality of the flames on the burners on the cooktop. Another telltale sign that the fuel level is very low.