On my 2003, the wire to the third brake light on the back of the cab runs up behind the plastic trim containing the grab handle on the driver's side. I don't remember the wire color now. I'll try to get it and post it later. You can splice into this wire and run a wire to an electrical plug for your camper shell. I wired to a two way switch so I can select either the stock or the camper shell brake light, depending if the shell is on the truck or not. The switch is under the dash, out of sight.
You'll probably want a receptacle or two, like the power receptacles in the dash, in your camper for an electric refrigerator, spot light, radio, or some other device. This same circuit can power the dome lights. Install an inline fuse on a battery terminal or the power module terminal and run at least 12 gauge and preferably 10 gauge wire to another pin on the electrical plug you used for the brake light. Use a 20 ampere fuse. Run the ground wire for the camper shell through the same plug so you only have to disconnect one electrical plug when you take off the shell.