Washington state sales tax isn't always 10%.
Washington IRS will except the actual selling price when registering the vehicle.
We bought our Newmar in state, we paid the sales tax once and this year we paid $116.45.
that stays the same.
When you buy out of state, then bring the motorhome into Wa. you will pay the difference between what you paid out of state when you bought it, and what Wa sales tax would be.
If the out of state tax was more than Washington tax, you won't get a refund, but you won't pay more tax here. (done that too.)