Holding Tank Dumping
Between black and gray, use the black water flush if you have one. Then the gray will flush out all those remnants too from the exit area and flexible tubing.
PS: don't use your fresh water hose OR the potable water supply tap for the black water flush. Either use a black/gray hose or a garden hose you brought along that you will not use for potable water. Use the sewer flush/rinse tap they hopefully have at the dumping station for this and for rinsing the sewer hose.
Land of 10,000 Lakes
2016 Forester MBS 2401R
2016 Forester 2401R