In a medium bowl, combine salad dressing, onion, lemon juice, red pepper, garlic, taco seasoning, Cajun seasoning, and black pepper.
Rinse turkey; pat dry. Place tenderloins in a plastic bag set in a large bowl. Pour marinade into bag; seal bag.
Marinate in refrigerator for 4 to 6 hours, turning bag occasionally.
Drain tenderloins; discard marinade. In covered grill, arrange medium-hot coals around drip pan, then test for medium heat above drip pan. Place tenderloins on grill rack directly over drip pan. Cover and grill for 15 to 18 minutes, or till steaks no longer are pink. Makes 6 servings.