When we bought our '72 airstream four years, it was registered in Alabama, which does not title a trailer that old. The PO had an email from the Alabama DMV that stated that the State of Alabama does not issue titles for 1972 trailer RV's. That email, along with the signed over registration and bill of sale was what we needed to get a title in Minnesota.
Try contacting the Vancouver DMV and see if they will send you a letter or email stating that they did not issue a title for the trailer, just a registration. Perhaps the PO can help you get in touch with the DMV.