Welcome to the SOC (Sunline Owners Club). Your coupler does have a small hole in it. Its purpose is to have a small pin inserted to keep the latch closed. They are sometimes in the shape of a printed capital D. Others are rectangular. The pin itself is round, approximately one/fourth to five/sixteenth in. in diameter. You can get them at hardware stores, RV dealers, etc. RV dealers have replacement light fixtures available, too. You could do internet searches for the marker/clearance lights, too. I believe your camper roof is aluminum. I have used Dicor. Others may have a better suggestion.
Paul and Nancy
1992 T-2251 - Our first Sunline
2007 T-2553 - Our second Sunline
2008 Chevrolet Silverado LT