I use one of these camping: https://www.igloocoolers.co.uk/igloo/bmx/52/cool/box/
A day or two before we leave i’ll put two or three frozen 2ltr bottles of water in it to chill it right down. If you pre-chill anything you plan to put in it in the fridge first this will also help.
Then anything frozen (pre-made chilli, curry, bolognese etc) goes in alongside chilled drinks (beer) and any perishables on top(dairy, meat etc)
I then dump a load of ice cubes in to fill any gaps or if you have ice blocks, shove them in to fill in the gaps.
In can be a bit of a pain to get to things, but if you pack wisely and limit how often you open the thing, you can keep things suitably cold for up to a week.
I also use the drain to empty out any water as everything melts and top up with more ice (from the supermarket) or re-frozen cool blocks if the campsite offers that facility
I find going in and out for drinks all the time is the main reason for the contents melting. If you can get a smaller coolbox specifically for drinks, this will help the food stay colder for longer.
The other tip is to put a wet towel over your coolbox, the evaporation of the wet towel will have a cooling effect on your coolbox.