You can learn about Read and Write in XML files here Processing XML Data with XmlReader and XmlWriter (Silverlight)
So, I assume that you want to save the content of richtextbox on XML File, right?
<richtextblock x:name="rtb" xmlns:x="#unknown" />
Code behind, something like this:
var contentRich = this.rtb.SelectedText;
After the code above, save contentRich
into your XML.
Hope it helps.