RadWindow may not be able to access the content page due to cross-site origin limitations in the browser, and the src attribute of the frame is not updated by the browsers when navigation occurs inside. Thus, if you need the current URL, you would need to access the iframe's window object as shown here: http://www.telerik.com/help/aspnet-ajax/window-programming-calling-functions.html. Here is a basic sample that can be added on the page where the RadWIndow is declared:
wnd = $find(
//wnd.get_contentFrame().contentWindow is the window object of the page inside the iframe
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