I90 all the way to the Seattle area is an obvious choice. But from there, you have several options, depending on your taste for urban freeways, streets, and ferries.
Sequim is on a peninsula, so getting there requires crossing a large body of water, or going around.
US101 follows a coastline all the way from Olympia. While level, it's somewhat curvy and slow, but pretty. I84, I5 skirting Portland might be better for this than I90.
WA16 and 3 to 101 crosses 2 major bridges. I think Tacoma Narrows is toll-free north bound. But traffic through Tacoma can be slow (take WA18 from I90). There may also be rush hour traffic in the Bremerton/Silverdale area.
I'd be inclined to take I90/405 to Lynnwood, and then WA104 to the Edmonds ferry. But that involves a ferry cost, which is much more for RVs. And depending on the time of days 405 can be slow.
A north route, WA20 all the way to Port Townsend (with a ferry ride) is most scenic, and avoids all the urban freeway driving, but I think the overall time will be more.
Washington DOT, WSDOT, has good travel information, times, traffic maps and cameras. They also run the ferries. In my experience Google Maps travel times and route suggestions are also good, at least for this area.
