Plumbing Stack

Found something kind of interesting today. We pulled the trailer out to wash it up and put some things away in it. We had it up, but didn't set the bathroom walls up. DW noticed that something looked weird thru the opening. The stack was not connected where the top elbow heads to the exterior wall vent. Don't think it ever could have been connected. It is about 1.5" short horizontal and 2.0" short vertical. Guess I'll be pulling the external vent cover and seeing if I can cut and repatch things together. Guess it kind of vents, but it brings external air directly under the sink.