My best advice would be a standard cooler, but using dry ice in addition to the regular ice. We went camping for over a week, had a chunk of dry ice combined with a block of regular ice, and everything stayed cold. The dry ice lasted more than a week, and kept the regular ice going for that long too.
I found our dry ice at a grocery store out in WY. You could call ice companies, listed in the yellow pages, to ask if they supply it.