20# propane cylinder is roughly 4.7 gallons when full (80% capacity is full)
That is roughly 427,000 btu's (91000 per gallon)
Furnace 20,000 btu so you have 21 total hours of furnace
20# exchange cylinder is the same DOT approved as your 20# cylinder
Only difference...that exchange cylinder MAY not be full (4.7 gallons) it may only have 4 or less) and it's price will be higher then a normal refill charge
CONVENIENCE
And yes you can use it, refill it keep it refill it etc.
Just check mfg. DATE on collar.
DOT cylinders have to be 're-certified' 12 yrs after mfg. date---so don;t get one that is close or out of date
Remote and off grid spot.......
What are you going to use to maintain battery voltage :H
That 12V DC Furnace is a HOG