Arrange onion slices in the bottom of a slow cooker; top with pork tenderloin. Sprinkle garlic evenly over onion and pork. Pour orange juice, vinegar, and lemon juice over pork. Sprinkle cumin, grill seasoning, salt, red pepper, bay leaves, and lemon zest over pork.
Cover and cook on Low until pork is very tender, 6 to 7 hours. To serve, slice tenderloin into 1 1/2-inch slices, arrange on a platter, and drizzle juices over top.