Crumble hot dog buns into a bowl and allow to dry out, at least 4 hours.
Preheat oven to 350 degrees F (175 degrees C). Butter a 9x13-inch baking dish.
Melt butter in a skillet over medium-high heat; saute onion until tender, about 5 minutes. Pour onion and butter over hot dog buns; add enough water to moisten the mixture and stir well. Season bread mixture with sage, poultry seasoning, salt, and pepper.
Arrange chicken legs, chicken thighs, and chicken breasts in the prepared baking dish. Pour 1 can cream of chicken soup over chicken; top with bun mixture.
Bake in the preheated oven for 40 minutes. Pour second can of soup over stuffing and add enough milk to moisten the mixture.
Continue baking until soup is bubbling, chicken is no longer pink at the bone, and the juices run clear, 10 to 15 minutes more. An instant-read thermometer inserted near the bone should read 165 degrees F (74 degrees C).