Any compressor fridge will draw more power when first starting up, compared to when its running stable on the thermostat. The average power draw (from both 240V AC and 12/24V DC) is listed in the specifications for each model.
If you are concerned about conserving power when starting up, a good tip (if possible) is to put your fridge on 240V power for 24 hours before leaving on your trip. That way when you put your fridge in your vehicle to run on your battery source, the fridge is already cooled to your desired temperature for your food and drinks.