I don't have frog experience yet but maybe some ideas from the aquarium world will help.

Insulate the enclosure. Drape blankets, towels, a thick coat or whatever to maintain the heat already there.

If it's moveable and you have an alternate heat source like a fireplace, then move the tank into that room. Not to close, don't want cooked frogs.

If you have a gas stove or camp stove you could heat some water and put hot water bottles against the tank. Again, not too hot.

There are battery backups from the computer world which could run a heater for a while. Another step up would be to use a generator.

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