Here's some good general advice:
1. If you're going to travel in the West, learn to drive in the mountains. Avoiding the Rockies, the Cascades, and the Sierras altogether is pretty hard to do.
2. Until you do get comfortable, make your major mountain crossings on interstates. Vehicles of all sizes and shapes deal with those mountain passes every day. You should be fine.
Don't sweat I-5 from CA to OR. Siskiyou Summit (and to a lesser extent, the stretch from Redding to that point) is the only place that can even be sort of dodgy. But given its bottleneck nature (you kind of HAVE to use that route if you're traveling from northern CA to Portland/Seattle), it's well patrolled, maintained, and plowed when necessary.
Getting over to I-5 is a doddle. Work your way over to Needles, then shoot over on I-40. The only obstacle on that route is boredom.