A SnoMaster fridge can also operate on the starter battery, so a double battery system is not really necessary. Running the fridge overnight is also not a problem. Depending on the size and condition of the battery and the type of fridge, running the fridge efficiently is possible for several days. To be safe, however, you should set the battery monitor to the highest level of 11.8 volts while running your fridge.
Again, everything depends on the circumstances. At cold outside temperatures, a diesel engine requires a lot of power for preheating and starting, but a car with a petrol engine and manual gearbox can be pushed easily, even if the battery is relatively low.