The Redwoods!
The Redwood National Park is commingled with several state parks, and are managed collectively. So stopping at a state park visitor center is about the same as the National Park visitor center and vice versa. They all share information.
Highway 101 goes through the Avenue of the Giants, with the giant tree groves right along the highway. A side trip to Fern Canyon is well worth it. It was used in a scene in Jurassic Park. There is no 'trail' per se, as it follows the creek bottom and floods annually. The 'trail' involves stepping over logs, crossing sandbars, and wading the creek in places -- but the canyon is spectacular with vertical walls of different fern species. Getting a motorhome to the trailhead may be problematic, but your car will have no problem.
The Forest Moon of Endor in the Star Wars saga was filmed at several locations in redwood groves. One of the main ones is directly across the Smith River from the Jedediah Smith campground on highway 199, a short distance off 101.
The Oregon Coast is scenic the entire way. The state has done a good job with oceanside state parks and catering to RVs. Tillamook has the cheese and ice cream factory open for tours (and free samples).
In WA, don't forget Olympic National Park. The Hoh Rain Forest is beautiful. Highlights include the Hall of Mosses trail (old growth forest draped in many different moss species). It rains a lot there, even in summer. Forks is reputedly the rainiest town in the US with over 200 days per year of measurable rain.
