You may want to look at your internal piping inside the counters. I believe FWC should have instructions on how to drain your pipes, and (I think) they started recently installing the piping to remove low spots and have a low point drain.
On my 2006 I removed the cabinet front and took a good look. I open the water tank drain on the back of the camper and the pulled the drain/anode plug in the water heater. That took care of most of the water. There was a small low point in the piping near the water heater inside the cabinet. It is PEX piping and somewhat flexible and I was able to reposition it so the water tank fitting is now the low point in that area. The last area I have to drain is the outside shower fittings accessible through a cabinet door. These can be removed by hand, drained, and put back on with some care, no tools required really. I find this fairly easy to do and I prefer dry pipes to RV antifreeze in this small system. I jog the pump a bit to help remove water from it as much as possible. I have found there is not enough water left to be a freezing problem. It is worth a look to see what you have and how it works.
Propane and appliances are off. I have a solar on top and as long as I do not cover it, it keeps my battery topped off, even in winter. Else I plug it in to the house. If you cannot keep it taooped off in teh camper while in "storage" bring the battery into your garage and put it on a battery maintainer or remember to periodically charge it (monthly).
I usually close up the windows and top vent and put is Dri-eze crystals in a container set in a plastic pan to catch caustic overflow. Around here there is so much condensation on open windows and vents it rains inside (on the vent interior or windows interior) if I leave them open. That was immediately followed by mildew. The dri-eze and closed system has worked well for my boat and camper for several years. Especially if you put a tarp over the top. If you have power and want to open the vents (in a way to keep the vermin and bugs out) consider a small fan to help move the air and minimize mildew chances. There are also RV/boat winter heaters, some with fans that help keep the air moving and dry(er). Look at RV centers (Camping World in Mt Vernon is not too far from you) and West Marine. Remember opening the lower access doors as vents makes a nice doorway for mice and cats.
If I put a tarp over, I try to place PVC pipe over the top, in combination with the load bars I have, ensure ventilation of the roof. The hard part is snow removal and rain puddle prevention. With my FWC metal roof I have elected to not cover it during the winter. My previous camper and boat had rubber or sunbrella roofs and it was better to keep them dry. Each spring and fall I have to lean off the algae that accumulates on the roof (mostly along the tracks and solar brackets, vent)in normal usage so covering for winter does not remove that effort.
You can apply 303 anytime, I prefer when the temperatures are not too cold. I store the camper with the top down, sides clean and dry. It should stay dry. Mildew primarly accumlates in wet area on the dirt on the surfaces. Clean dry siding fabric should not accumulate mildew.
During the winter I periodically check in on the camper and open the door and air it out. I leave the top down. I tend to bring in the towels and blankets and other soft goods. Given teh succes sof keeping the inside dry I have not needed to bring in the cushions. Bringing them in is one less thing to worry about though while you establish your success baseline.
I have visions of using my camper a bit during teh winter to spend a night on the coast winter storm watching (like Kalaloch). Since I tend to leave the camper on the truck longer these days, Covering it is not convenient and I keep it mostly ready to go, so closing it up and Dri-eze is the simple route for me. I use bottled or jugs of water rather than fill the tank for simple winter excursions. When I do take it off, I use jack stands supporting 4x4s under the floor with the camper jacks carrying minimal weight.
You might see my camper in the area as I freqent Walker Valley ORV park east of Big Lake and one of my daughters is in the dorm at WWU so we visit and deliver "stuff" periodically.
- Mike