Come up 580 across the bridge to 101 and take 101 all the way to Gold Beach. My wife & I did Vancouver, WA to Santa Rosa for our first trip with a 36ft 5th wheel - no problems at all. I'd never towed a 5th wheel before this trip and it was a piece of cake. We hit the coast around Lincoln City, Oregon. I've been told to stay the hell off SR 36. I wouldn't bother taking I-5 north to Williams and then coming across Hwy 20 to Ukiah - too hilly compared to 580/101. Why go thru the hills when you can go
around them?
I just realized you are leaving at 4PM..... on a Friday. You're gonna get hosed no matter which way you go. Traffic on 101 from San Rafael to Santa Rosa is going to suck. Going I-5 to Hwy 20 adds a lot of miles & time. You may want to come up 680 to 80 and then cut over on Hwy 12 thru Napa/Sonoma. If you do take Hwy 12 you want to go around Sonoma & Glen Ellen on Arnold Drive - you avoid going thru town. You'll be in a bit of traffic from where Hwy 12 merges on 101 - I DO NOT go thru this interchange with the trailer, it's bad enough with just a car - to Windsor and then you'll have pretty clear sailing. How far are you trying to go on Friday evening? There is a WalMart in Windsor - Shiloh Rd, and in Ukiah - see it from the hwy.
Call me at work 707-577-3559 if you need/want more specific directions - I live in Santa Rosa.
Brian