Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Add zucchini and yellow squash cover, and steam until tender, 2 to 3 minutes. Drain well and and transfer to a bowl. Add bell pepper and onion to the bowl with the squash, mix, and set aside. Whisk eggs in a large bowl. Add mayonnaise, Cheddar cheese, salt, and pepper. Add squash mixture and 1/2 the butter and mix well. Pour into a 5-quart slow cooker. Add a layer of sliced tomatoes. Top with breadcrumbs and dot with remaining butter. Cook casserole on Low for 4 hours.