Strangely, the cheaper the better. The more expensive kettles are usually fast boil and as such have an appetite for amps. The cheap ones as a rule tend to require more time to boil, as they have less powerful heating elements. Just take a look at your local electrical store and compare the wattage on the kettles, the lower the better. You don't need a specific "caravan kettle" on most sites, but there are always exceptions to the rule unfortunately, just check up before you go to find out the output of the hook-up.