We have just come back form our first camping trip where we used an outwell powered cool box (bought from go outdoors for about £45). I can honestly say it was great, the milk butter cheese etc was kept really cool but by far the best was the cold booze!
just make sure u use as low wattage as you can so as not to overload the campsites leccy and fuse the whole site ( it happens ive seen it, campers using a 3 kilowatt kettle !!)