[Carl n Susan]

I-5 over the grapevine is really not a problem. Yes it is a long 6% grade but nothing worse than other stretches of I-5 (Mt. Ashland grade near the southern Oregon border). Thousands of 18-wheelers traverse it each day. Just remember to control your speed coming down and you will survive.

I am not a fan of interstates and I-5 & Hwy-99 are two of my least favorites. If you are pressed for time, they are expedient. Remember the speed limit in CA while towing is 55 (or so). Map programs assume you can travel at max speed and you can't. Consequently they tend to underestimate the travel time But if you have a couple of more hours to spare, and you don't mind driving an extra 50 miles (350 v. 300), I would take a different route.
The route I would take is Hwy-41 to Hwy-46 and then Hwy-101 to Oxnard. Below is a map of this route. These are all good roads (I just drove most of them 2 weeks ago with the RV), scenic and interesting.