Unless you know someone at the WA DMV who has brought a camper down from BC before you are in for a surprise. There is ton of paperwork that needs to happen as campers are not titled in BC. If you can get the paperwork and get it back across the border you will need to file for a lost title, get a state patrol inspection, and wait 3 years to actually get the title.
I was lucky and had a buddy who buys cars at auctions in BC and knows brokers to handle the paperwork mess. Even then there was a lot of back and forth phone calls, and we had to wait at the border coming home because the border guard had no idea what was going on...