Hi Jean, If you plan to use your furnace a lot in the cold, I would start with 2 full propane bottles, set temps a low as comfortable, and be in the habit of checking the automatic change-over green/red switch, located on your regulator - daily. If it switched to red, I would then switch to a small electric heater, instead to preserve propane in the 2nd tank. Personally, if we have hook-ups and its cold, we prefer to use our small electric heater to preserve our propane. However, if you use a small electric heater, some units can draw a lot of power so watch what other high powered items you use at the same time. If you use too many high current devices at once, ie, electric heater + microwave + hair dryer + electric hot water heater, etc, it can trip the circuit breaker. Don't ask me how I know!
BTW, congrats on your new 5.0! -Bea