Bring salted water to a boil in a large pot. Add potatoes; cook until tender, but not mushy. Strain potatoes and flash cool in ice water. Allow water to completely drain. Pickle the onions: Combine dill-pickle juice and diced red onion, and soak for 1 hour. Strain the pickle juice and save the onions. Make the dressing: Combine mayonnaise, barbecue sauce, salt, creamy Italian dressing, black pepper and celery seed in a mixing bowl and whisk to combine. Combine potatoes, pickled onions and dressing in a mixing bowl, and gently mix to coat ingredients. Season to taste with salt and pepper.