One might be missing some basics in all of this discussion, which is how much food you're taking with you and how fast you're going to eat it to begin with. I've generally been feeding 1 man 1 dog with a Coleman 6-day cooler. Meat for him has often been the reason for the cooler, as I don't believe in dog food or canned food. In warmer climates we've typically gotten 5 day worth of preservation out of the cooler, using my "standard" 20 lb. load of ice. I keep the car in the shade with the windows rolled down as much as I can. One trick to going longer is to buy as many frozen foods as is reasonable. Some stores will sell chicken gizzards (which I feed my dog) frozen solid; other stores will fetch completely frozen ones out of the back of the store, if you ask. Also remember that cold water is still a refrigerant, you only need new ice when that water is beginning to warm up. You need water tight containers for you food though when it enters the "slosh" stage, to avoid hazards of defrosted meat blood mixed with the water for instance.
Anyways after 5 days we're going back to civilization for more food anyways. Can get stir crazy and desire to change camps as soon as 2 days, which is part of why I don't use a tent anymore, just the car. Could extend my time greatly if I wanted to, such as accepting dog food, but not clear so far that I'd want to. Consequently gold plated coolers sound like a waste of money to me.
Many coolers function by their size and how much ice you put in them anyways. Don't be too easily fooled. Those big "7 day coolers" you see, mostly work because they're big and you're putting lots more ice in them. Which means more money if you have to pay for the ice.