This has been discussed many times. The state matters: in California, it is illegal to put the license plate on the camper, the vehicle code is quite unambiguous about this. A camper is a load in the bed of a pickup, and the license plate must be attached to the vehicle to which it is issued, period. The 200 foot unobstructed view has clear exceptions: when you are towing a box trailer, the plate is not visible at 200 feet behind. Much more visible underneath a camper.
If a patrolman enforces his own interpretation, he is either having a very slow day or you have angered him in some way. With my previous camper I had a tool box mounted on a hitch extension right up against the plate. The rear plate was not visible, not even a little bit, even crawling underneath. I would have had to take the toolbox and hitch extension off to see it. I drove it that way all over the western states for a decade. Never a single question.
