As has already been said.See above.
To get maximum usage time it is good practice to cool the fridge down for 24 hrs beforehand whilst plugged into a Mains power supply and any items added prior to departure chilled in your kitchen fridge before adding.
I put cans , gin and tonic and bottles beer in beforehand and switch on the fridge and then add cold meats, cheese, milk etc: from the kitchen fridge 24 hrs later. Salad stuff etc: should go at the top and to the front of the fridge as the rear wall is where the heat exchanger is. So alcohol etc: against back wall as it has a lower freezing point. Also have some form of spacer on the base and back wall to ensure some air circulation.
It's worth getting a cheap fridge thermometer as well.