You will be a CA resident and not a WA resident right?
CA makes it very tough to have a vehicle registered elsewhere but it might be possible if you understand the details. Start by calling the CA BOE and reading Pub 52 and there may be other documents that you need to understand. As a CA resident I met the requirements to buy and keep a MH without paying any use tax. This may/may not apply to your situation.
And just so you know: DO NOT depend upon anything the DVM tells you. DO NOT depend upon anything the BOE tells you. The ONLY thing that matters is when the BOE checks to see if you owe taxes is how they interpret the documents at THAT time. If you understand and follow the requirements then you should be OK. I speak from experience as both the CA DMV and BOE gave me both incorrect and correct information on the phone. But I got it right and the BOE agreed.
Consider keeping a signed/dated log of each event that is relevant to what will be considered relevant for you situation. A binder with non removable pages, add receipts etc, whatever is needed for your "proof". Even consider having it notarized at points in time.
