Preheat oven to 350 degrees F (175 degrees C). Grease a large casserole dish. Place bacon in a skillet and cook over medium-high heat, turning occasionally, until evenly browned, 5 to 8 minutes. Drain on paper towels. Cook and stir potatoes and onion in the skillet until golden brown, about 10 minutes. Add bacon and diced tomatoes; stir until heated through, 3 to 5 minutes. Pour potato mixture into the casserole dish. Whisk cream of chicken soup, milk, and eggs together in a bowl. Pour over potato mixture in the casserole dish. Bake in the preheated oven until eggs are set, about 40 minutes. Sprinkle Cheddar cheese on top. Continue baking until cheese is melted, about 5 minutes more.