After seeing your video, I took a photo of how Sportsmobile West did my gray water drain (picked up my Sprinter April 2017). My plumbing looks like yours (90 degree ABS elbow glued to some sort of white plastic attached to the tank). So far no leak, but this is another point of failure I will keep an eye on.
Is it possible that your 90 degree elbow got hit from under the chassis and pulled it away from the gray water tank?
Ron & Barb - 2016 SMB Sprinter, 6 cyl, 144", 2WD, high roof