I received all my paperwork for the trailer from the state of Michigan.  Paid sales tax and registration as required.  Michigan and Washington have an agreement on taxing new purchases.  I stopped by the local department of license yesterday to register the rig at our home in Sammamish.  They decided that I need to pay an additional $1,250 plus in sales tax and registration.  I guess I got away cheap due to not having a motor in the rig.