Rice bug, If I read the law correctly, what it says is this. If you buy out of state( in your case Idaho) you would pay Idaho sales tax on the rig. Which Google says can vary from 6 to 9%. Washington state sale tax is 10% so you would have to pay Washington the difference.
However if one reads further, and this is the important part. As I read it, if you buy in another state, and use it for more than 90 days, in that state, then you will owe NO additional tax in Washington, when you register it. The additional tax would only seem to apply if you buy out of state and move to Washington in less than 90 days.
So buy it more than 90 days before you move is my thought. Or, did I not read far enough and RV's are treated differently?