I am so sorry you are dealing with the two illnesses, and that things are not going well pain-wise.
I am so sensitive to the weather as well, and I have to say I am inclined to believe that the pain is probably part of the RSD. But since I am not a doctor or know much about pelvic pain or MS, I really can't say otherwise besides that.
Have you talked to your doctors about all of this? In terms of helping to relive the pain because of the cold, I know a heating pad really helps me. Also, if you have to be on them or go outside, try slipping those handwarmers (the ones that activate when they are exposed to air) inside your shoes or socks- I know that helps my ankle and foot immensely if I have to be outside.
Hope those little tips help- I know it's not much, and I wish I could help more

Try and keep your spirits up, and do some things that make you happy. You are in my thoughts and prayers!!