Rinse chicken, pat dry with paper towels, and use your fingers to loosen the skin over the breast and thighs.
Stir butter and lemon pepper in a small bowl and insert seasoned butter beneath the loosened skin using a teaspoon. Push butter from spoon using the skin. Place lemon slice and garlic cloves into the chicken cavity.
Place chicken into a slow cooker. Rub skin with olive oil and sprinkle seasoned salt, thyme, and cracked black pepper over the bird.
Cook on High for 3 hours; reduce heat to Low and cook until chicken is very tender and an instant-read meat thermometer inserted into the thickest part of a thigh, not touching bone, reads at least 160 degrees F (70 degrees C), 2 to 3 more hours. If desired, cook bird on Low setting 6 to 8 hours.