Thanks everyone for your advice. Thought I'd give an update on our trip north from San Francisco up the 101 during freezing night time conditions.
Not only did we survive, we enjoyed every minute of it AND stayed toasty warm. We have a 4 year old, a 2 year old and a 3 month old baby and my wife was super concerned about the temps before we left. I purchased a Mr. Buddy Heater and packed an extra extension cord and splitter so that we could run two small electric heaters simultaneously. Turns out, after a 10 day trip, we'd only used one tank of propane despite the night temps ranging from 30-35 degrees. We only stayed in campgrounds with electrical hookups, obviously. It turned out to be a great trip, the kids loved it and the wife didn't complain about the cold once.
I'm not exactly sure how to post a picture, but you can see our adventure by clicking here
. We started in San Francisco and followed the 101 north to Washington and then came down through Portland on the 5.