Yes, they have to be kept above freezing till they harden.
General practice here is to use an flexible electric heating pad, with an insulation blanket - all night. The flex heating pads are often the type sold for auto oilpan heating, so they are real common in the auto parts stores - well here anyway
Another practice where power is not available is to use chemical hand warmers - also with insulation. Sporting goods stores have these. But they only last for a couple of hours so they have to be changed several times till the Chico hardens. Really a bummer job at -40F. Can't say I've ever tryed it at -60F.
For insulation blankets, ordinary fiberglass insulation held with tape, covered with visqueen(sp?) held with tape, works pretty well.
I have never heard of using rv antifreeze. I've heard of people using methanol or isopropyl alcohol, but I don't have any data on how well it worked.
The reason for knowing this is just another of the benefits of living in the Great North
carl