It is too late to edit the above post, so I'll put some more information here. Assuming the numbers I've looked up are accurate....
A fully filled 20# tank will have 4.7 gallons of propane. For two tanks that is 9.4 gallons.
91,502 BTUs X 9.4 gives 860,119 total BTUs. 860,119 BTUS / 20000 BTUs/hr gives 43 hours of burn time for two filled tanks.
Allowing some inefficiencies and not fully topped off tanks, say 40 hours. Your furnace will not burn all that time so the actual heating time will be much longer. A minimum of least 80 hours and likely much longer, depending on the circumstances.
As mentioned by bikendan, if you don't have a 120V connection then your battery power is the limiting factor. If you do have 120V, then even a small heater will dramatically reduce propane usage. Most people report the small ceramic heaters with fans to be the best.
With this setup, you turn on the heater and set the furnace thermostat to a bit lower temperature than you normally would. Both will work until the camper is warm, then the furnace will cut off and the ceramic heater will exclusively (and quietly) provide the heat as long as the heat loss doesn't exceed its capacity. At which point the furnace will cut in to give a big boost.
2012 Rockwood A122S
2008 Toyota Tundra Double Cab 4X4
Former owner of a 2002 Coleman Niagara GTE