The best way to keep your food cold is to take ice with you. With your cooler at the ready, you will need to fill it with blocks of ice, so that the contents of the cooler bag stay cold and fresh, and ready to eat.
In addition, if you are camping for a few days, then you can freeze meats and foods such as burgers, so that they stay cool for longer on your travels, or thaw out in time for you to eat them. Just make sure any foods you froze before putting the cooler are completely thawed before you cook and eat them!