you can't go wrong with an Oregon coast trip. If you continue into Washington state, the coastal roads are pretty good, but do avoid Seattle if possible.
You could cut across the middle of Washington from the coastal region to Tacoma, then skirt around on I-5. If you go into the Olympic Park, cut back south to Tacoma.
We've pulled our trailer on these routes with no problems. If you do tackle the big cities, plan ahead for EARLY morning or LATE evening travel and then it's not to bad. There is so much to see and do, that it's worth a little effort. Enjoy!!