Is your coolbox an electric one that you are going to use unplugged? If so, then I found that ours wasn't very efficient used in that way.
If it's an unpowered box then it will very much depend on the type and thickness of the insulation. Do you have a make and model you could Google for information?
We have a Coleman Xtreme 36 which we use for non-ehu trips and I pre-cool it with 2 x 2-litre bottles of ice the day before we go and then replace with 2 more bottles when packing with frozen/chilled food, milk, orange juice etc. on the day we go. There is usually still some ice in the bottles 5 days later. This is for UK camping - can't guarantee it'd be the same in the south of France.
As for ice packs, I can't say I've noticed much difference between them but then we don't use them very often.
the thicker ice packs last much longer than the thinner ones, our freezer has about 15 of them! depending on the heat at the time 2 was sometimes enough other times we used 4. I also used to freeze water/milk before we went which helped keep things cool. (use an electric one now)
Frozen 2 litre lemonade bottles of water work for us, as already suggested try freezing everything before you go. And try to get the box as cold as you can before you fill with your goodies and ice packs or bottle.
If I could offer a tip it would be to freeze three 5 litre water bottles and use them. Yes they are bulky but this means that they have mass and will take ages to defrost. They need at least 2-3 days in the freezer though to freeze properly.
When they are nearly defrosted in the cool box, we have bought packs of ice cubes from Lidl (89p per bag) and stuffed the ice cubes into the bottles, resealed them and put them in the cool box. This is handy when there's nowhere to freeze water by the bottle and where campsites will charge a small fortune for freezing small ice-packs. It also keeps your coolbox free of water from molten ice-cubes leaching blood from packs of meat etc as they get submerged and contaminating everything else that's in the box.
also plan your meals - we freeze the chicken which then acts as another ice pack and it's defrosted by day 2\3 and is ready to cook.
Also frozen sausages as they can be cooked from frozen anyway - basically freeze anything you can that is either safe to cook from frozen or your not in a rush to eat once you get there.
We often goto sites which have no facilities to refreeze blocks so tend to have to plan this way.
like the others, freeze as much as possible before hand, even a few pints of milk can make difference to the temp in the box, and use what defrosts first,
we use 2 large square ice packs and if possible try to get them refreezed on site, but most of our camps there are no facilities, so its frozen food and an occasional top up of ice cubes, which we put into bags that can be sealed up, stops the leaking
if its not EHO, you might also want to try the cold wet tea towel trick ( learnt it off here) basically take 2 extra t towels, soak them in cold water and put them over the top of the toolbox on the outside, replace with the next wet t towel as the first drys out
make sure you keep the box in the shade and if possible any breeze, we've done this and come home with stew after 4 nights that was still part frozen,
We are considering purchasing a Coleman Extreme coolbox. Are they worth the extra that you pay or should we just invest in an electric one that can run off 240 and 12 volts DC.
------------- Pitdiver
Welsue's other half and also a ,Camper, Sailor and a nearly Outdoor Person.
The flexible inside of a wine box, filled with water or brine and placed at the top of the coolbox is fairly effective, nearly as good as the lemonade bottle trick.
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Topic: Coolbox advice
so followed advice.
