Old-Biscuit has given the answer. It is not illegal in California as long as the vehicle is licensed as a housecar or camper which would include a motorhome or even a conversion van.
Many many years ago I did have a little first hand experience with this. I had a van that had been converted and was licensed as a camper. Was returning from a week long camping trip and was pulled over in a small California town for a "burned" out tail light ;). I was not drinking however I did have 3 passengers in the back who all had open beers. There was also a passenger in the front who like me was not drinking. It was late at night and we were just passing through and of course we looked suspicious with our long hair and van. Officer came to window and went through normal license registration routine. Shined his light in the back and saw the individuals with the beer. He immediately asked for everyone's id. The basic rule is as long as it is behind the driver compartment then it is legal in a camper. After calling all the id's in he came back and found an excuse to search the van. Long story short after practically stripping the insides he left us sitting on the sidewalk to put everything back. He did leave us with a warning to not let him see us in "his" town again or we would be arrested. Being the youngster I was I couldn't help but ask him, for what? Like a scene from a movie he just looked at me and said he'd find a reason. And yes I was able to and did pass a field sobriety test.