Spray the inside of a slow cooker with cooking spray. Season the tri-tip roast with garlic pepper seasoning on both sides.
Heat olive oil in a large, heavy skillet over medium heat, and sear the roast on all sides until well browned, about 5 minutes per side. Place the tri-tip roast into the slow cooker, fatty side up.
Cook and stir the garlic and onions in the hot skillet until golden brown, about 10 minutes, and season with the ancho chile pepper, cayenne pepper, and black pepper. Place seasoned garlic and onions over the roast in the cooker, and pour in the white wine and crushed tomatoes. Cover the cooker, set to Low, and cook until tender, about 8 hours.
Remove roast from the cooker, and cut off the fatty layer with a sharp knife (tender meat should cut easily). Place the roast back into the cooker, and shred meat with two forks. If desired, remove some of the liquid from the meat. Stir in cilantro and serve.