On my older GM trucks (70's and 80's), the camper wiring was a long pigtail that came up from the frame between the cab and bed and hung over the front bed rail. My dad's old GM truck was the same way. These were all factory Camper Special package trucks.
It does work like that, but can also interfere with mounting a camper shell or hard tonneau cover or cross-bed tool box, lumber rack, etc. It is easier to hook up when the pigtail hangs over the bed rail though, rather than having to crawl under the truck to plug it in down by the frame, where it may also get covered in mud.