Charlie,
I would suggest: NV-372, CA-127, I-15, CA-58, CA-14, I-5. This route is about 20 miles further than 138 but, it has less traffic and more 4 lane road. CA-138 is almost all 2 lane road. This is the route we took in November.
Unless you are meeting the Escapees in Castaic you want to consider California RV Resort in Acton. It is 9 miles further from Pasadena than Castaic but a nicer newer park.
http://www.calrv.com/
I assume you are currently at the SKP COOP in Pahrump. We are planning to go there next weekend so we will just miss you. Are there are a number of open sites?
Grant aka BirdingRVer