This plug is for fans that clip onto the rod that goes into the spot to the right in your pic. Sorry i dont have a picture handy, but you can find them on amazon. They usually ahve a light in them as well. I serchrd hybrid camper fans and they came right up, they look to be running about 30 bucks. Hope this hleps!
Below in the link is what the fan is. In the late '90s and early 2000s they used to include one fan for each bunk with new hybrids. When we traded our '03 Kiwi 17a for the 2013 X20E, I kept mine thinking they would be spares. The sales guy asked me if we still had them and recommended we keep them because they are no longer included with the new units.