Like Todd says, it's really not a problem here in the PNW. There can be 'spells' of cold air into the teens but normally not a big problem. I did bust my transom shower head due to freezing during one of these spells. Water tanks and holding tanks are very unlikely to freeze. Putting a West Marine Air Dryer w/Fan on the cabin sole will be the equivalent of that 25W light bulb and poses no problem for fire hazard.

Full winterization is not really preferable because you might actually want to go out some days in the winter, so empty water tanks for instance wouldn't be preferred. (I figure a nearly full tank will never go 'solid'. If a boat were kept on the hard here, I would do a more conventional New England type winterizing.