Spanking her makes me a little nervous - it's just repeating the pattern of pain she knows too well. Is her therapist helping at all? Perhaps she needs more intensive therapy from someone familiar with trauma and RAD? I really do feel for you, this is a really tough situation for both of you.
Has she talked to you at all? Will she color or draw? This little girl has got to get all these confusing, scary feelings out of her. I wish I knew the right thing to say...my heart goes out to both of you.