If I was driving (car not camper) to a festival + I didn't have much spare cash after the cost of the tickets + it was going to be 3-4 days + one ring primus stove ...
I probably wouldn't bother with a cool box/bag
I'd get some supplies that I could keep in the car and go back to the car once/twice to pick them up. Tinned tomatoes and chickpeas/red kidney beans/vac pak flat breads mainly.
What I would have in my tent:
olive oil
small cooking onions
garlic
root ginger
cous cous/quinoa
red lentils
tomato purée (thickening & conc flavour)
chorizo or other dried cured sausage
pine nuts/raw cashew nuts
dried milk
apples, small oranges, couple of lemons
spices - black pepper, salt, chilli flakes, basil/oregano, paprika (thickening agent), cumin seeds, turmeric, mustard powder
ground coffee and tea leaves + infuser
baby bels/waxed cheeses
jar of black stoned olives in brine
dried fruit
supermarket fresh veg for as long as they lasted: peppers, courgettes, aubergines, green tomatoes, leafy stuff, mebbe some green bananas
One pan, a wok