Bring a large pot of lightly salted water to a boil. Cook elbow macaroni in the boiling water until cooked through but firm to the bite, 8 minutes; drain. Rinse with cold water to cool completely; drain.
Mix macaroni, tuna, eggs, tomatoes, onion, and cucumber in a large mixing bowl.
Whisk salad dressing and milk together in a separate bowl until smooth. Beat sugar, mustard, pepper, and salt into the dressing mixture; pour over the macaroni mixture and stir to coat.
Cover bowl with plastic wrap and refrigerate at least 2 hours.
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