If you'd be running the generator solely for the fridge, use propane.
If you'll be running the generator for other reasons such as air conditioning, use AC power. If it's just a quick run of the generator (make coffee or microwave lunch) it's probably not worth the hassle of switching back and forth manually. It generally makes no sense to run the generator just to power an absorption fridge, and doubly so if it's a propane fueled generator.
If the fridge has an automatic setting (many do), that will switch between propane and AC power based on whether or not electric power is available at the moment, and it usually works out best just to leave it on automatic most or all the time.
While there are a great many variables, a handy rule of thumb in the absense of any better information is to guess that an RV fridge uses a pound of propane a day on propane power, maybe a little less in your this case as it's a quite small fridge. It's not a large consumer of propane relatively speaking.