It is illegal to keep a loaded weapon in a vehicle in California.
The exception would be a parked RV that has become a temporary residence for the evening, or at a campsite etc
Once you stop someplace for the evening, outside of various state park regulations which may prevent it, you may have a loaded firearm. Until then handguns should remain locked up, and unloaded.
Long guns must remain unloaded.
An error in transporting a handgun is a felony for someone from out of state (because there is a felony enhancement if the gun is not registered with the DOJ, and only residents can register.)
This means you should keep handguns unloaded in a locked case in the RV. A locked cabinet or glove box near the driver's area is not acceptable even though the gun is unloaded. Once you put it in a locked case it can be kept wherever you wish.
You may have loaded magazines, but if they are concealed and you have access to a handgun then the loaded magazines themselves are considered an illegally concealed handgun per case law. But if the handgun is in a locked case you don't have ready access. This primarily applies if the handgun is legally unconcealed outside of a case, just keep it in a locked case.
Restrictions on long guns are limited. You can have them anywhere in the vehicle. They can be concealed by something or unconcealed in the vehicle. Ammo can readily be available, and the above magazine stuff only applies to handguns. The long gun however cannot be loaded in a vehicle.
You can have ammo in a side saddle, and loaded detachable rifle magazines (though you will need to understand the complex "assault weapon laws" to have such a firearm in California) in a vehicle.
You are stripped of immediate firearm self defense while traveling in California, but regain it each time you stop someplace legal for the evening.
So while the RV is a parked home you can have loaded guns as it becomes a temporary residence, it is no longer a vehicle for firearm purposes. While it is a vehicle in use you cannot and are subject to the same rules as a car.
How do you known when it is one and not the other? Gray area. Personally I would lower the jacks and have the keys away from the ignition with the engine off. Parked in a campsite or RV park is also going to be much more legally defensible than a parking lot, and the side of the road or highway is very likely to have it still considered a vehicle.
Some of the nicest places to visit as a tourist in California are also State Parks, which typically have their own separate and even more restrictive laws (not just rules, but laws as certain rules trigger state laws.) Which can mean even the above may not be allowed.
