Easiest route to the redwoods from the north with trailer

In my limited experience, the best part of the Redwoods is between 199 and 299.

There's a coastal canyon (Fern) in Prairie Creek State Park that would take you back north to coastal Vancouver Island, with ferns and large Sitka Spruce. The big Redwoods are a bit further inland (e.g. Lady Bird Johnson Grove).

South of Eureka is the Avenue of the Giants, which has a more commercialized feel.