For all intents and purposes, you can't defend yourself in a vehicle with a handgun. You legally can shoot in self defense, but you'd have to get the gun out of the trunk or a locked case (can be just a gun rug with a little luggage lock, but
not the glovebox or console) first.
See these pages. Brief and worth perusing.
http://www.chp.ca.gov/html/answers.html and
http://ag.ca.gov/firearms/travel.php
As of now, NPS doesn't allow carry, so in Yosemite and Death Valley, it doesn't do you much good to have a gun, either.
California law does, however, specifically allow you to keep and carry a loaded gun, openly or concealed, while occupying any temporary residence including a hotel or campsite (except where prohibited, like a campground in a National Park
). So, if you're planning on staying in a sketchy part of town to save money, by all means, bring a gun.
With the possible exception of a very rare bad neighborhood here or there, Monterey and San Diego are not dangerous places, and as much as I understand feeling naked without a gun if you're not used to going unarmed, I've honestly never needed one in town. I've lived in coastal Orange County just to the north, and here in San Diego, for my whole life.
LA is a different ball of wax, with notorious gang capitals near havens for the wealthy. It's hard to avoid the sketchy areas, though one can certainly stay the hell away from Compton and other such places. Still, I've never been robbed or shot at, in LA. I avoid the place, usually. Stress, traffic, and a culture of *******s really isn't appealing.
I have occasionally felt like I wanted a gun when camping in a remote location. But again, your outdoor destinations preclude that.
Bottom line? I generally don't travel with a gun unless I'm hunting or something. Given that you'll be in National Parks, and in relatively safe places, I'd be more concerned about having the thing stolen than having to use it. Auto burglary is a far more common menace than violent attacks, here.
And I second:
http://www.calguns.net is a great resource.