Hwy 12 from Yakima to 101 is a great way to go.
You could also stop along Hwy 12 near Packwood for a night and take 123 north to Mt. Rainier Nat'l Park (or check out it's campground from that end - not sure of the length of sites)
You could also take the road from Randle - 131 south I believe - which will take you to the back side of Mt. St. Helens Nat'l Volcanic Monument. Quite a view! Iron Creek National Forest campground is not far off Hwy 12 and is very nice to explore Mt. St. Helens. Don't take your RV beyond Iron Creek. We stayed here with our 40' MH and also used it as a base for Mt. Rainier with our towed vehicle. I believe it accepts reservations, also.