It's RMS, but not for the extra reasons given in the other answers. It's nothing to do with the power calculations, or the time lag of fuses, or the temperatures of conductors.
Of the several options (peak, RMS, rectified DC average etc), RMS is the one that's been chosen.
Once a standard way of making the measurement has been chosen, all the other ratings can be adjusted to suit. For instance, if we measured voltage and current as peak, then we would simply rate our kettles at twice their present wattage as peak watts, and our cables at 1.414 their present current as peak amps. No domestic user 'knows' or even cares whether a 2kW kettle is 'really' 2kW, what they need to know is how fast it will boil, and can it be connected to this socket? Experience and the ratings will tell them those. The measurement scientists will understand what's going on, and make the appropriate calculations.
As it happens, RMS is convenient for the measurement scientists, it does reduce the number of conversion factors. But convenience rarely has a look-in around electricity supply technology, look at the conversion factors needed to use AWG rather than mm2 for cables?