Temperature settings are subjective! There are a number of primary issues to consider;
1. Is it a combi fridge freezer or single compartment. (my combi when set to -18 in the freezer has a fan that circulates into the fridge compartment and is not adjustable. Without the divider panel it gets treated as a single compartment fridge OR freezer.)
2. What are you putting in to it re food and drink, i.e beer and softies can sit at a higher temp safely, whereas milk needs to be colder. And, yes I know, who wants a warm beer, lol.
3. Ambient temperature in operating environment. A fridge/freezer works a lot harder in summer than winter.
4. How much battery power capacity do you have. You need enough to cater for worst case scenario.
5. Type and frequency of replacing used battery capacity. i.e. solar, DC-DC, mains etc.
The key thing re settings is; The lower you set the temperature of a fridge or freezer and the warmer the environment, the more power it will use and the more you need to replace.
The other important consideration is; For food safety reasons a fridge should be set to 5C or colder and, for frozen food, particularly things like fish n chicken at least -18C.
With so many different fridge brands, sizes and combinations and power sources, there is no definitive answer other than if it's safe and works for you, fine.
