Create a simple Parse XML rule to match the SOAP fault structure. Map ErrorDescription element to a property of your choice in the Parse XML rule. Once you have the fault XML in pySOAPFaultDetail property, call Apply-Parse-XML method in an activity.
I can only provide a few high level ideas on this (I don't have the time to try these at the moment in detail to see if they are feasible).
1. Can you use a PARSE-RULE on the Property - to map the bits of the XML you need to another Property ?
2. Are able to create a simple XSLT file that extract the textnode of the XML you need ? If so - PRPC has some built-in Activities that allow you run an XSLT over an XML document - you will need to search PRPC for the Activity names (search XSLT) - I can't remember them off the top of my head I'm afraid.
3. Maybe you can just treat the XML as 'text' : as search for a specific string (using PRPC Libraries - use the 'Expression Builder' (etc) to browse through them) - rather than extract a particular string ?
4. You might also be able to use a REGEX to extract the string you need from the XML if you are reasonably sure of the variability of the format of the XML ?
As I say : I haven't tried any of these - they may or not be viable - just wanted to give you some ideas...