WA will charge you tax on what they feel the value of the coach is. It makes no difference how old the coach is or what you really paid for it. If you paid $50K and they say it is worth $100K, you pay tax on the 100k
What they will do is use your paperwork showing how much tax you paid to whatever state. If you don't have any proof of paying sales tax, they will assume you paid zero and you will owe whatever the tax board feels is due based on the evaluation of your coach. You will get credit for the amount of tax you have paid to another state
If WA tax is less than what you paid, you get no refund.