If you start Skyline to the Sea from the Castle Rock SP parking lot, and not from Skyline itself you can avoid hearing the road noise from Hwy 9.
There is road noise for the first 3 or 4 miles from Waterman gap to the point where you start climbing China Grade. It is still an awesome hike: waterfalls, wildflowers, earthquake faults, and the ocean at the end.
For remoteness, I have never seen anyone at the trail camp inside Butano SP. Portola SP is scheduled for closure at the end of August,. You can reach its trail camp from Skyline Blvd. I have never seen any other backpackers there as well. There are also two trail camps in Pescadero County Park.