I did not want to put a plug in my bed either.
I run my extension cable over the tailgate from the bumper connector to the 5er plug. I added a ring to the extension cable with Ty-wraps. The ring is located on the cable inside the bed close to the top of the tailgate. I use a rope with two snaps on it. One for the ring, the other in my case goes to one of the chain rings on my goose neck hitch. I'm sure I could have found a place on my fifth wheel hitch, but this was convenient. The rope keeps the cable outside the bed in light tension. My stock tailgate also has a plastic cap over the top of the tailgate. Extra cable lays in the bed by the tailgate out of the way.