We live just up the coast from you on Van. Island. It does get humid here quite often! In the past, i have used Dry-Z-Air in my Roadtrek, with good results. We have also experienced times where things were really moist inside. When that happened, it was later found that the rear door had developed a leak, coming inside and getting everything moist, raising humidity. We now leave the roof vent cracked open (with a vent cover), and the front windows down a crack. We also have this little fan thing, designed for marine use, that keeps things bone dry. It works great.
linkIt sounds as if you need to check all over for water ingress. It isn't usually a concern, just being parked over the winter, but leaks sure are! Just look at how many units are parked all winter at RV Dealers, all over the country! They seem to make it ok.