Getting the propane to flow in the very cold weather has always been an issue for propane powered vehicles. Heating the tank with oil or coolant would work but I just had another idea.
Why not pressurize the tank with a little methane, or natural gas? Since it is effectively incondensible it will pressurize the tank. It will eventually be absorbed into the propane so it will be gone by summer, or one could just bleed off the vapor side of the tank before it warms up. You could flare it off but if you do it in a couple of short bursts on flat ground even that would be unnecessary.
Only a small precharge would be needed, I think.