All I can tell you is we drove from Chicago to the SF Bay Area, in November, on I80.
NEVER again towing a trailer!
December would be worse.
Can't suggest a north to south route from Ontario but you need to take I10 west or I40 west and then head north to San Rafael, once you get to California.
Dan-Retired California Firefighter/EMT
and Zoe the Wonder Dog(R.I.P.)
2016 PrimeTime TracerAIR 255, pushing a 2014 Ford F150 SCREW XTR 4x4 3.5 Ecoboost w/Max Tow Package