Well, If I wanted to head South from Northwest California and entirely avoid the entire SF Bay area, I would probably divert East on Highway 20 at Ukiah to I-5 at Williams (85 miles), proceed South on I-5 to either: 152 West (at Los Banos) to San Juan Bautista on 101: or Hwy 41/46 West to Paso Robles on 101. Your return West from I-5 to 101 is 50 to 75 miles.
If you can stand SOME of the SF Bay area (but not pull through the city streets of SF itself), I would say drive down 101 to The Richmond Bridge (580) (it is free in the Eastbound direction, about $15 with the trailer in the Westbound direction) merge from 580 to the 880 at the Bay Bridge interchange, stay on 880 down the East side of the bay to 101 in San Jose.