In a small bowl, whisk together vegetable oil, white distilled vinegar, water, orange juice, onion powder, salt, mustard powder, sugar, Splenda, and poppy seeds until the dressing is well combined and slightly thickened.
Taste and adjust seasoning if needed, then transfer the dressing to an airtight container and refrigerate any leftovers.
Divide the washed and chopped romaine lettuce between two large salad bowls.
Top each salad with half of the sliced grilled chicken, drained mandarin oranges, blueberries, strawberries, pecan pieces, and pineapple.
Drizzle the desired amount of poppy seed dressing over each salad and serve immediately.