Have you filed a police report? Since you haven't mentioned that, I will assume you have not. Here in Oregon, and in California, it is a crime to rent property and fail to return it. In the case of something this valuable it is a felony.
The sooner you can get it reported, the better. I'm assuming you have the VIN of the trailer, and once it has been reported it will be entered in the national system just as if it were a car. If he removes the plates and sells it, when the new owner tries to register or title it, it will show up and you will recover your trailer. But you have a couple of hoops to jump through first, and you need to start immediately.
I've attached the appropriate California law. If you have a friendly relationship with your local police agency, they might figure out a way to get it entered even before it is officially considered stolen, such as entering it as a vehicle of interest, which is just as good but only temporary. Good luck.
JP