Sounds like a great trip. We'll be repeating it next summer. I'd run up I-5 to Williams, CA. At Williams, cut over on CA-20 and go around Clear Lake and through Lucerne until you hook up with US-101 north of Ukiah. Take US-101, the Redwoods Highway, north (there's a nice rest area we generally have lunch at just north of Longvale) and you'll see the redwoods open up beautifully right after Leggett. Garberville is cool, we bought a redwood bear carving there. I'd then head up to Eureka, CA and base camp there for a few days. Be sure to eat at the Samoa Cookhouse
when there. It's all-you-can-eat family style dining in the original cookhouse of the logger's camp. You will get stuffed but you can take a walk on the beach just south of there to take in the sunset and walk it off after you're done.
Eureka is also a great base camp to explore the redwoods areas in the area. We'd stay at the KOA. To the north of Eureka is Orick (no they don't make vaccum cleaners there
). You can camp right along the beach in that area and if you take the side road into Fern Canyon State Park (leave the RV behind) you can see Roosevelt Elk right on the beaches and take the Fern Canyon hike, which is where they filmed Jurrasic Park II. Be sure to eat at Rolf's Restaurant, which is right at the corner of US-101 and the road into Fern Canyon. You can see elk from his back porch or you can eat it on his dinner plates and he is a very good cook.
Heading north you can see more of the Redwoods and coastline, including an old WWII radar station. Eventually you'll get to Crescent City, where I'd stay at the KOA again. From here you can explore the Jedediah Smith State Park, which is where the Star Wars speeder bike scene and Ewok village was filmed. There are some great nature trails in there that are easy hikes.
Just north of the CA-OR state line is Brookings, OR. If you stay at Harris Beach State Park you'll be right by the ocean and the wonderful coastline of Oregon will begin to open up for you. You can beachcomb in the tidal pools at Brookings and drive up the coast to look at the great scenic overlooks in the Samuel F Boardman series of parks. Once you get to Florence, you'll be in the midst of the coastal sand dunes. You can rent dune buggies, hike quiet nature trails, hop on a horse to trail ride the beach, etc. I'd stay at Woahink Lake RV Park.
Traveling up the coast you'll get to one of our favorite spots - Cannon Beach, OR. Stay at Cannon Beach RV Resort. There's lots of great eating places in Cannon Beach and Haystack Rock is awesome, especially at low tide when you can walk out to it. I walked out one morning at a very rare extremely low tide and actually got to walk into a cave in the side of the rock, which is huge. The tidal pool exploring is awesome in this area and you'll see all sorts of different colored shellfish and marine life you never even knew existed.
Traveling north into Washington you can cross the Columbia River at Astoria via a very long bridge. Astoria is also the home to Fort Clatsop, the winter encampment for the Lewis and Clark expedition. If you like history, stop and see this. Once in Washington, the coastline isn't as spectacular as Oregon so head inland to I-5. Mount St. Helens is an awesome site to see. I climbed it last summer but the volcano is now closed to climbing because it's been "burping" recently. We have stayed at Columbia River RV Park in Woodland. It gives you the best central location to see both sides of the mountain. The south and east sides are accessed through Cougar and Yale. You can drive up the Spirit Lake Memorial Highway to go into the blow-down area and you can walk into the lava tubes at Ape Cave from this side. If you head north to Kelso, you can take WA-504 through the Toutle River valley all the way to Johnson Ridge Observatory. From here you can look right into the crater from the blown out side (don't worry - it's safe
). The visitor's centers along this route will tell you a great story about the 1980 eruption. Be sure to see the movie at Johnson Ridge. When the screen opens up at the end you'll see the crater right through the window and you'll have a huge lump in your throat. this area is very underestimated and is worth seeing.
Heading north, I'd go west to hook up with US-101 again at Aberdeen. Then head north on the coast (you can park right on the ocean just south of Kalaloch), stopping off at the Hoh Rain Forest area of Olympic National Park before continuing on to Port Townsend. You can stay at Fort Worden State Park and camp on the beach next to the old coastal artillery emplacement (which make for some great exploring). Then base camp out of there to see Port Townsend's old victorian architecture as well as the Hurrican Ridge and Sol Duc Hot Springs areas of Olympic Nat'l Park. Then I'd take the ferry to Keystone and drive up Whidbey Island to Anacortes. At Anacortes I'd stay at Fidalgo Bay Resort and take the Island Adventures whale watching cruise to look at the Orcas in San Juan Strait. You can also take the day trip ferry to San Juan Island or drive the toad down to Seattle. I found Seattle to be the typical big city, with not that much to do. However, the surrounding areas offer plenty. In Seattle I'd stop at Pike Place Market, take the monrail down to the Space Needle (expensive if you take the elevator up), and maybe even consider the Underground Seattle tour.
This trip will take about 3 weeks so that'll give you some frame of reference to base your trip on. Regardless of what you decide to do, you will find this to be a great trip. There is lots to see and do. Your biggest problem is to find out how to fit it all in. Sorry this post is so long but I get over enthused about this area and there's tons of stuff I haven't even mentioned.