The most common cause of a furnace or water heater not operating in colder temperatures, but operating when the ambient temperature is warm, is the LP Regulator.
In cold weather the diaphragm in the regulator can become stiff, limiting the output pressure, which is critical for appliances that draw large amounts of LP. The Gas Valves on items like the furnace and water heater will not open if the supply pressure drops below a certain point. This is a safety design.
The stove burners and oven, which have a very low consumption may operate fine.
The system pressure must be measured with a device (gauge or manometer) designed for the low pressure. Normal pressure gauges will not work.
A "farmers" check of the regulator operation can be done by turning on the stove burners and observing them while you turn on the furnace or water heater. You should see a very slight, momentary, dip in the height of the burner flame and it should quickly return to the original height. If it does not return to the original height, the regulator is suspect.
Also on RVs which have the propane cylinders on both sides (not physically together in one location) there is an additional regulator at the tank opposite the tank where the changeover/regulator is located, and that regulator could be the defective one. You can eliminate that remote regulator by ensuring you are on the changeover side when testing.
2013 Rockwood 8289WS
Mods: Central Vac, PI EMS-HW50C, 2025RV, (2) SCS225 Trojans, PD9260, Awning Remotes & LED Lights, Eliminators, External Water Filter,, Intellijacks
2008 RAM 3500 Cab&Chassis Hauler Bed, B&W Companion Hitch
All My Modifications are at this link: