In a large slow cooker add juice reduction, remaining orange and lime juice, all zest, olive oil, garlic, oregano, salt, pepper, and chili powder. Stir to combine. Add tenderloin and turn to coat. Spoon marinade on top of any exposed meat. Cook on high for 4 hours or on low for 6 - 8 hours, turning meat several times through the cooking process. Meat will easily pull apart when ready.