Highway 101 ! With the exception of Hwy 1 in California, the most beautiful coastal highway in the USA.
I came up from California on 101 and took it all the way to Seattle. Just a great ride along the coast. Whale watching, small towns and fishing villages. I thought the Lewis and Clark National Park near Astoria was interesting. Fort Columbia State Park in Washington on the Columbia River is worth the visit, also Cape Disappointment State Park. But just driving along the coast is the real attraction.
FWIW, I pull a 36 ft fiver and had no problems finding places to stay - but I do make reservations about a week ahead of time (fulltimer). Check out Eagles Nest Resort in Ilwaco, Wa. Not really a resort, IMHO, but it is in a good location, lots of trees and has pull-thrus, etc. Just make sure to tell them how long you are. Good grub at the Port of Ilwaco.