The dash heater/AC in a diesel pusher is just too small to heat (or cool) 40' of motor home. For our first big winter trip, I made a curtain to hang behind the seats. The plastic track can be purchased from Camping World that goes across the ceiling. Accompanying attachment snaps get sewn onto the top of the curtain. I made it in two halves, with a tie-back for each side when not in use. Make it floor to ceiling, and close it when you're traveling. It made all the difference in the world. We didn't have any cold air leaks from the front -- just a 40' long metal tube in which the heat from the dash was simply lost. The curtain kept us from having to run the furnace going over the Rocky Mountains in November, or from needing to run the AC in the Sacramento valley in summer. I found fairly heavy upholstery-type material that matched our interior. Hem it on all four sides, and sew the snaps to the top. Done.
Road cams look clear to me. I would get some chains and roll straight down I5.
An unusual situation in the Siskiyous at the moment, whereby the Siskiyou summit is clear, but you get into snow a few miles south, at the Oregon/California border. But check TripCheck.com and look at the cameras in northern California, especially the southern-most camera at Weed. That one shows some very nasty conditions. And the worst winter conditions I've ever seen in dozens and dozens of trips up and down I-5 were right in that area, near Mt. Shasta. The cameras right now (at not quite 1:00 pm) show one lane kind of clear, and the other lane completely covered in snow and ice -- a nasty situation with so many big rigs plowing down the right lane.
Anyway, who knows what things will be like by the 27th. Just don't be fooled by checking ONLY the cameras in Oregon. Yes, the Siskiyou Summit is the highest, but it won't necessarily be the only place you can get into some really stinking weather.
First off, it would be most helpful if you could tell us what vehicle you're in or towing or whatever, and your ultimate destination.
Interstate 5 is one of the first roads to be made passable when big snows hit. If there's a really big dump of snow and they can't get I-5 opened, then 199 to the coast down to Highway 20 is a good winter route. If you're pressed for time and can't just wait out a storm to take I-5, that's the route I'd take. I live on Highway 20, and can tell you that it will take around 2 hours to get from 101 to I-5, depending on what you're traveling in/with. Do NOT under any circumstances take 36 to Red Bluff, unless it's the middle of the summer and you're driving a sports car and want a challenge.
Depending on where you want to end up, Highway 20 may not be your best option through the maze of the Bay Area. Again, if we had more information, we might be able to offer greater assistance.