Our generator ran on propane and kept the refrigerator running on demand. We would keep a soda bottle half full of water and frozen. Before leaving for any length of time we would flip it so the ice was up. If we came back and the ice was on the bottom, then we knew the refrig. was off for a time and that anything in the refrig. was suppect to damage.
This is a good idea for an freezer or refrig.