When it comes to draining the calorifier turn your taps being used for the water to run out of to the hot position and let it run until no more water comes out, and do it with all taps individually and not forgetting the deck shower ⛄️❄️
If the boat is ashore for the winter, I use the deck shower hose as the drain and pull it out as far as it will go and leave it dangling overboard where it then also acts somewhat as a Syphon with its end being below the underside of the hull. Don’t forget to remove the shower head from the end of the deck shower, just in case any water remains within it as the cost of replacing the head will not amuse !!
To combat the risk of frost damage I’ve installed a couple of mains electric pole heaters, one located next to the calorifier and the other next to the water pump and pressure vessel. It does mean leaving the shore power supply connected and switched on which normally I try to avoid, but with global warming our winters in parts of the UK don’t always get so cold.