Till,
that's true. Not all VIs are supported in LV RT as it states in the LV RT User Manual (7.0, page 1-7, 1-8). In your case the XML VIs require the MS XML DOM parser that is getting installed with LabVIEW on Windows and is unfortunatelly not available on the RT platform (Pharlap).
I don't know if there is a XML workaround or if NI will integrate other OS independent XML parsers which would run on RT.
You might consider to use the .ini file approach for the RT platform - I assume this should work.
Roland