In the Yoder Toter teardrop, there is a fan that pulls air out of the cabin (ala Hunter) and exhausts it through galley vents. The vents seem to exist for a place for the exhausted cabin air to exit.
My "Drain" also acts like a vent. It is just a PVC elbow mounted to a hole in the floor and a drain screen mounted to it to keep things out. Sorry I don't have any pictures of it (teardrop is offsite in storage) I can get one in a month or so if you can wait.