In California it is illegal to make a copy of your plate for any reason including putting one on your camper because the original is blocked by the camper.
In California it is illegal to put your license on anything other than the vehicle it's intended for. That includes any cargo you are hauling that would block the view of the plate.
Let just ask; can you see your truck's plate if you are towing a trailer or 5er 200ft behind your truck? No, of course not. So why would towing that blocks the view be any different than hauling cargo that blocks the view? You don't take your truck plate and put it on your trailer. Sure, a trailer has a plate, but that doesn't preclude the visibility of the truck's plate or even if there is a plate on the truck.
I've driven campers off and on for over 25 years in CA and have never moved the plate off the truck and put it on the camper and I've never been confronted by any LEO for doing so.