The real number to read is the Wh rating, in this case they publish it at 850. But wait, there's more. Life expectancy at 80% discharge is 800 cycles, which for Lithium is a bit weak. Little better than lead acid, but most of the Lithium Iron batteries are rated in the thousands of cycles. And that 80% of 850 is 680 Wh of usable power. Another issue I see is you state you camp in 2-8° nights. Looks like the battery has a very narrow temperature band of 10-40°. Lithium Iron has temperature limitations as well but let you run down to 0° without issue and most will allow discharging down even lower with only charging being limited to above 0°C. I don't see that battery being a good choice. Very high price for very little energy with very limited usable temperature limits and a pretty short life.
So lets go to the fridge. I see 2 conflicting numbers in the specs. One states 45 watts, the other is 5.0 Amps at 12V, which is 60 watts. As others have stated the power consumption of the fridge is a HUGE variable. Think of it more as a home A/C than a fridge for a moment. When it is fairly cool outside you don't need to run the A/C much at all. When it is really hot outside, you run it a lot. The amount of time on vs. off is the duty cycle. On a cold day and there is no A/C run, 0% duty cycle. But a really hot day, people in and out all the time, A/C never turns off, that is 100% duty cycle. The A/C takes about the same amount of power while running (not really, but close enough). The portable fridge is the same way. On a hot day, it runs a lot. On a cold night, runs a little. Fill it full of warm food and ask it to cool it off, it will use a lot of power. Lets take worst case (not what you describe, but a starting point). 680 Wh and the full duty cycle is 60 watt of power. 680/60 = 11.3 hours.
OK, published 45W power consumption, I assume this accounts for the duty cycling. Under what conditions, they never say. But you want answers provided with the limited specs you offer, this is what you get. 680/45 = 15.1 hours of run time. Not very impressive is it?
Taking guesses, because that is all we can do given the lack of data, mixed in with what you did provide for usage plans and temperatures, you might get 24 hours on that battery. Maybe. That is a SWAG with what I have seen in the past and what you provided. A little sunshine mixed in, inside a closed car during the day, it will go down to the 11 hour mark really fast. Fridge kept in the shade, good ventilation, kept in cool ambient temps, back toward the 24 hour mark.
The power consumption for the fridge looks fairly typical. It is the battery pack that is undersized. If you added a good solar system to compliment the battery it could work. But it is still a very over priced battery for the performance. And shy of sleeping with it you will be hard pressed to keep it in a happy temperature zone.
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