Don't know the California section, but 101 through Oregon has many miles of 4-lane roadway. Not a problem for your rig. Once over the Astoria bridge into WA, it's mainly two lanes, but quite manageable. Logging trucks are frequent users.
If you're not planning on sightseeing the Olympia to Burlington area, an option back across to I-5 would be to go off 101 at its junction with SR-20 to Port Townsend and take the ferry to Coupeville, then SR-20 to I-5 at Burlington. You'd save a couple of hundred miles. The only tight spot on SR-20 is the Deception Pass bridge. It was a CCC project and was built to the road width standards of the day. It's used by big trucks every day, but is not very wide.
Frank Damp, DW - Eileen, pet - female Labrador (10 yrs old), location Anacortes, WA, retired RVers (since Dec 2014)