1. Heat charcoal or gas grill for 2-zone cooking; direct heat zone about 400°F and indirect heat zone about 275°F. Season pork loin with grill seasoning; let stand 20 to 30 minutes.
2. Boil sweet potatoes in salted water for 5 to 7 minutes to parboil; drain.
3. Place pork loin over direct heat zone, turning every 2 to 3 minutes, until each side has grill marks. Move loin to indirect heat zone.
4. Place cast iron skillet over direct heat zone. Add olive oil and onion; cook until onions have softened. Stir in sweet potatoes and cinnamon; cook for 3 minutes. Add apples and sage; cook for 3 minutes.
5. Place pork loin on top of sweet potato mixture; drizzle maple syrup over pork, cover pan with aluminum foil and move to indirect heat zone.
6. Continue cooking until internal temperature reaches 145°F. Remove from grill and let stand 5 to 10 minutes. Slice pork and serve with sweet potato mixture.