You can buy them at the auto parts stores in Alaska. They are of different voltage and have a double sided tape on them which sticks to the oil pan underneath the vehicle. They were of different shapes, rectangles. Several different sizes. Can get another kind for under the batteries. Can put more than one on for a larger vehicle. we run the cords to a multi outlet at the front and then plug the thing in at 15 or below if the unit is parked outside. Works like a charm.
Not sure how long you have lived in Tok, but we were in Nenana Alaska for 13 years and the temperatures are much the same as Tok. By late March, the chance of -40°F temperatures is not high between the lower 48 and Tok. Since the OP mentioned "returning" to Tok, I am guessing the OP is outside for the winter.
The years we lived in Nenana we used propane for heating domestic water and never had it quit working. During the real cold spells, down to the -70°F range, it would take longer to heat water or dry clothes but would do it. Now at our stick house, I had a high pressure regulator on at the tank and ran 30 psi pressure to the house, underground, then once inside the house, I had a regular low pressure regulator at the 1/2 lb pressure rating. If we had enough snow on the ground, I would try to keep some shoveled up around the 300 gallon tank we used. At the super cold temperatures the ground tends to stay warmer than the air so the tank can stay a bit warmer if snow covered. Would not apply to an RV propane tank though. The cab heater should keep most RVs warm enough to drive even if the propane didn't work. Just keep the RV winterized and no water in the tanks and you should be OK. You can always stop at a road house to spend the night if needed and can't sleep in the RV.
Moisture, from the fuel, freezing in the regulator on an RV tends to cause more problems than will the actual propane not vaporizing.
Most years we would leave our RV parked and winterized till mid May, give or take a week or two. From March on, the weather normally is great for snow machine trips, dog team trips with tent camping or aircraft flying on skis. Never had time to play with our RV till later in the year, about the same time the river boat came out of storage. LOL
On edit: All of the heating pads/tanks/freeze plugs, I had for my vehicles were 120 volt, not 12 volts. Most vehicles the are used in the Interior of Alaska will have a 120 V extension cord on the front to be able to plug in. If a person can find a 12 volt pad of some sort, I guess you could run it off the alternator while driving. A few of those and running the furnace at night would have caused me to have had a depleted battery by midnight. LOL Most campgrounds will be closed in the north in March so finding electric plug ins may not be easy.
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This a problem I've never heard of. All my life heated houses, ran pickups on propane. We can get below 0 in the Texas Panhandle. I've seen as low as neg. 24. Propane did not freeze, and nobody worried about it. If this is really a problem it I would want to know more.
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You guys have had very different experiences than I have. While normal furnaces or fridge on propane have never. Caused a problem, I had a tank freeze up yesterday. I was in Waco TX at +44 F and had 1/2 a 40lb tank get covered in ice. The result was a end to the fuel flow. My engineer brother-in-law ( after all it was Christmas day ) tells me it had something to do with the speed with which the propane was being pulled from the canister. He.rattled about some "heat exchanging" stuff which I tried to listen to, but kids were crazy...well you know. The fuel draw was mainly a 5500 Onan fueled by LP. I know this thing burns fuel fast....but that fast?
We refilled at Flying J and the frozen tank only took half the volume of the empty ( non frozen ) tank.
Any ideas? Did my brother-in-law nail this one? If so, hog did the freeze up happen? Shouldn't the regulator manage the draw rate?