Based on a picture (attached for reference) I found of the sewer compartment of a 2650 here is my best guess.
The bracket comes from the center area to support the black tank drain on the right. The two halves are typically held together with a nut and bolt, the hardware may have come off. I would bet the metal control rod you mention is the other half of the bracket.
The strap and bracket are thin black straps about 1” wide.
If the “vent” you speak of is a 3” diameter foil looking tube just hanging there, it is a duct from the furnace to keep the tank compartment from freezing.
2017.5 Forester 3051S
2019 Jeep Trailhawk toad