We are in Seattle too. After having RV's in the Northwest for 12 years, I finally have THE answer.
I cover our camper with a Heavy Duty plastic tarp, and, run an electric dehumidifier.
I got the humidifier at Home Depot for a little less than $200. About once a week I undo the end of the tarp to get into the camper and empty the water in the tray.
The weekly dumping is a pain in the butt. However, it is well worth it. This last winter our camper was much dryer than the previous 11 years. No moisture. No mildew. No cabinet door swelling. It was fabulous. No need to "air it out" before the first spring trip. I like it. You will too.