I bought an 80 litre box from Techni Ice for $149http://www.techniice.com/english/iceboxes.asp
Only used it once so far and am pretty happy with it
Despite hot weather (high 20s - low 30s), not keeping it in the shade
and opening it 2 or 3 times a day, after 4 days the 4 X 2L frozen bottles of water
and the 6 techni ice sheets were at least half frozen still.
4th night stayed in a cabin (wife didn't like idea of tenting it in a thunderstorm)
and refroze a couple of bottles and ice sheets
so can't really say how long originals would have lasted but
7th day most things on the bottom were just frozen, everything else was cold enough
Only prob I have with it is its height, at 20 inches it only just fits in the boot of the Commodore
and the Camper Trailer is approx 18.5 inches deep, so I have had to unscrew one the under bed
hatch hinges in order to fit it in comfortably for transport.