Ooops. I promised to talk about that and didn't do it yesterday. My apologies. Now first off I want to make sure you know my boys were 3 and 6 when they came, so this is some what speculation. I believe potty training with a child with RAD could be difficult for several reasons:
Other diagnosis (FAS, SID now being called SPD) can make it difficult. If you suspet your child has either of those you might want to read up on it. http://www.nofas.org/ and http://www.sensory-processing-disorder.com/ . I would also discuss it with your pediatrician to make sure all phsycial conditions had been ruled out.
If there are no other diagnosis and your child physically is fine it could still be very difficult to potty train a child with RAD. They missed stages of development during trauma that effected the way their brain works. Therefore they are physically and emotionally immature. Of course, there are the emotional control, fear based issues that could exist.
I would focus, if the child is a preschooler, instead on snuggling, tons of physcial touch, eye contact, snuggly type games like patty cake, this little piggy, singing together, dancing together. That sort of thing. This site has great ideas http://www.attachmentdisorder.net/Nurturing_Activities.htm Work on being attuned to their feelings and emotions and saying back what you think you feel. Take turns copying making faces at one another. These activities, as weird as it sounds will get you to the potty training faster than actually focusing on the potty training.
I'm sure it is frustrating to have a child that is old enough to toilet train and not having it go well. We cannot control what goes into them or what comes out of them unfortunately. So we have to focus on what we CAN do instead.