To give some "meaning" to the above comments about extremely cold weather use of propane tanks, here is a link that provides all the data necessary:
http://www.dvorsons.com/Magikitchen/...guidelines.pdf
On page 2 is a chart that gives the "boiling capacity" or "BTU AVAILABILITY" of a 30 pound propane tank. At 20*F a half full tank will provide 45,360 BTU's to the regulator. At 0*F that same tank can only provide 25,200 BTU's to the regulator. What that means to an RV'er with half full propane tanks is that the regulator can't provide the 30,000 BTU's of propane needed to keep the furnace propane supply above the input requirement. So, the furnace will either shut down (flame out) or not be able to maintain the heat output to the hot air registers.
A 20 pound propane bottle can only provide about 70% of the above output, so it is even more incapable of powering the furnace.
Most of us wouldn't want to be in our RV at -15*F, but that same half full 30 pounder could only provide 5,670 BTU's to the regulator. If you think about it, the "HE burner" on the stovetop is a 8,500 BTU input. Essentially, a half full propane tank can't provide enough propane to the stove to keep one burner operating at full efficiency. Without the ability to "boil off" propane to supply the input gas necessary to operate the furnace, it's pretty much elementary whether there's a Bilzzard Package, Polar Pak or any other "special cold weather capability".
For someone who is going to try to survive in an RV with exposed propane tanks, there are "tank blanket heaters" available that would help keep the propane warm enough to vaporize in extremely cold weather.