The "bottle in a sock" idea is a very old system, just a variation on the "chargul" (excuse the spelling, it may be wrong) These were issued when we were in the desert in the early 60s, they probably were about before then, they were a bag made from from a waterproof material and held about a gallon of water, which we hung on the front of the vehicles so that the got a bit of breeze passing over them, we then just had to keep dowsing them with water when we stopped. We also hung them in the tents, leaving the flaps open for the draught. But I suppose nowadays with the cutbacks the Army cant afford them, they probably cost all of 50p to make!! Also when we had to use desert well water it tasted terrible, so we always put in a drop of Lime cordial to give it a better taste! But they did the job "Cooling by evaporation"THe sock was tested out in the gulf at the start the current conflict in Kuwait summer by my partner. The water doesn't have to be cold to begin with, it works on evaporation and it was how the soldiers kept the milk fresh!
It works for me every time and it means I make my little contribution to helping the environment but not running a fridge at home (which I turn off anyway whilst on holiday) and wasting yet more energy in a caravan. Really you want to use a caravan to enjoy the environment rather than destroying it by burning aviation fuel flying off to some foriegn destination all the time.
Tankie