In a small bowl, stir together garlic salt, poultry seasoning, and black pepper. Season all sides of tenderloin; set aside.
Add white wine, plum sauce, and cornstarch to a slow cooker; whisk to combine. Place both tenderloins into cooker, and sprinkle remaining spice rub over the top.
Cook on HIGH for 4 hours.
Let tenderloin rest 25-30 minutes before slicing. Slice 1/2-inch thick slices at an angle.
Serve sliced tenderloin hot with plum sauce drizzled on top.