In 3 1/2- to 4-quart slow cooker, place turkey breast tenderloins. Sprinkle with seasoned salt and pepper. Add onion and water.
Cover; cook on Low heat setting 6 to 7 hours.
Remove turkey from slow cooker; place on cutting board. Shred turkey with 2 forks. Measure liquid from slow cooker; add enough water to liquid to measure 2 cups. Return liquid to slow cooker. Add taco seasoning mix; mix well. Stir in shredded turkey. Cover; cook on Low heat setting 1 hour longer.
To serve spoon about 1/4 cup turkey onto center of each tortilla. Top each with lettuce and cheese; roll up.