To the OP:
rent a Private Mail Box (PMB) at a UPS store or other parcel store that rents mailboxes. You can not do this at a Post Office.
The legal street address will be something like:
1234 Main St PMB 5678
(where 5678 is your box number)
The WA Dept of Licensing (DOL) computer system cannot handle "PMB" (at least not in 2011), so do the following:
Change the address on a credit card bill and cell phone bill (or bills of your choice) to be:
1234 Main St Apt 5678
or
1234 Main St # 5678
If you have paperless billing, turn it off so that you'll receive a paper copy at you new mailbox address. Then take the bills to a DOL office and change you license and/or address.
The UPS store or parcel store where you are renting the mailbox will tell you that they don't care what is listed on the street address as long as you box number is on the address label - so that they won't have to look you up on their computer. It can say SUITE, APT, FLOOR, BLDG, UNIT, #, PMB - they don't care.
I used the # sign instead of PMB for the address on my license.
I have found that for all of my change of addresses, I start with PMB on the street address or separate line. If that gets rejected as invalid syntax I use APT. If that gets rejected I use #. etc., etc.
** EDIT **
One last thing... Rent you mailbox in a county in WA that does not require emissions testing in order to register a vehicle. For RVs it is pretty much a joke and a pain in the butt. Avoid making your domicile in the counties in this link:
http://www.emissiontestwa.com/E/Findlisting.aspx