You'll be getting into the rainy season and the end of the tourist time so you probably won't have a problem finding sites.
If you have a problem with making reservations for Seattle, I'd recommend driving up the Olympic Peninsula from Portland and staying at Port Townsend. Fort Worden State Park is great and it's a neat little town with lots to do in the surrounding area. You can make a day trip in Seattle by taking the quick ferry from Bainbridge Island (you can drive your towed to the ferry landing). The ferry will take you right to Seattle's waterfront by the Aquarium. You'll be by Pike Place Market and you can pick up a neat monorail that will whisk you to the Space Needle. It makes for a fun day.
Another ferry day trip while staying in Port Townsend is to take the short drive to Port Angeles and take the ferry to Victoria Vancouver Island. You'll be right in town and you can take a public bus out to Butchart Gardens.
Yet another day trip is taking the ferry with your car to Coupeville and making a nice day trip up to Anacortes, WA
Port Townsend is a nice base area. Olympic National Park is also nearby. When ready to leave the area continue on 101 around the Peninsula which will take you down the Washington and Oregon coasts - a must.
Full-timed for 16 Years
. . . Back in S&B Again
Traveled in a 2004 Newmar Dutch Star 40' Diesel