Mexican Layered Salad Recipe

4 cups torn romaine 1 medium red onion, thinly sliced, optional 1 large cucumber, halved and sliced 3 medium tomatoes, chopped 2 medium ripe avocados, sliced or cut into 1/2-in. chunks 1 each large green and sweet red pepper, chopped or sliced 1-1/2 cups mayonnaise 1/4 cup canned chopped green chilies 2 teaspoons chili powder 1/2 teaspoon onion powder 1/4 teaspoon salt 1/4 teaspoon garlic powder 1 cup crushed tortilla chips 1/2 cup shredded cheddar cheese