In a small bowl, combine the ground sage, garlic powder and onion powder. Put the pork tenderloins into a slow cooker. Season with salt and black pepper. Sprinkle the seasoning over both sides of the pork.
Cover the slow cooker. Cook on low heat for 6 to 8 hours.
One hour before the pork is finished cooking, combine the brown sugar, balsamic vinegar, soy sauce, water and cornstarch in a saucepot. Whisk well to combine and dissolve the cornstarch. Bring to a simmer and cook, stirring occasionally, for about 5 minutes or until it thickens up. Pour the sauce all over the pork tenderloins.
Cover the slow cooker. Continue to cook until the pork is cooked through (145 degrees F on a food thermometer), about 30 minutes to 1 hour more.
Slice or shred the pork and serve drizzled with the sauce.
