The outlet you describe is for running lights into the bedroom wings of the pop-up. Most of the ones I've seen clip onto the upper support rod (we always called them "candy canes") for the canvas and end panel. Here's a link to see a sample of what of these light fixtures look like.
RV Bunk Light by BargmanŽ
Because those sections of the pop-up fold down, it would be impractical to permanently install lights. The constant bending and compression by the canvas from opening and closing it would surely break the wires (and maybe even the fixture itself) over time. Besides, you have to remove the "candy canes" anyway so you would have to unplug the lights too.