We trailer year round in the PNW also. I watch the weather and don’t often winterize. Our boat is stored at the house and uncovered but with good southern and SW sun exposure for good sun heating on the blue hull. The shore power cord LED can be seen from the house to verify power.
Normally use just an oil filled radiator on the floor of the v-berth to combat mildew and maintain temp inside the boat with all compartments and cabinets open. I try to keep the interior temp above 35-38 minimum. During past winters I placed multiple outdoor temperature probes in the boat (engine bilge against hull, floor of head, floor of v-berth) and monitored them in varying temperatures with simple min/max recording stations and found a heater setting that seems to work.
If below 28 at night I put a round disc low output heater in the engine compartment of our Classic that is powered by a cord run from the head outlet thru the strbd cockpit storage and pulled to above the muffler. That way the cord can stay in place when we do go out.
During our ‘cold’ week or two is the only time I throw in rv anti-freeze. RV anti-freeze goes into the head and holding tank, shower sump, wash down pump, and engine. I only use water in the transducer cup and pop the transducer out during cold weather. Thoroughly draining the water tank with the bow raised and lowered to the limits of the tongue jack as well as draining the water heater, blowing out the lines and leaving them disconnected has served us well these past years. The only pain is getting the fittings tight on the hot water tank each time. I now keep a couple spares on the boat just in case one won’t tighten back up as happened this spring.
For the one week a year when it doesn’t get above freezing, I add a 3rd heater with a fan that runs on a separate circuit and cord with a fan to help move air as a last resort.
Cold prep takes me an hour or so and 4 gallons of anti-freeze when required.
So far this winter nothing more than the oil filled heater has been required.
I am currently experimenting and monitoring with keeping the water tank full and the hot water tank turned on but have not made a decision on the benefits and drawbacks of that yet as opposed to the security of draining and prepping yet.