I do not have much eexperience connecting directly to the Salesforce Communities data. We utilize the Salesforce connector at Tableau but it sounds like this not an option for your scenario. Does the data get published in JSON, XML or HTML? If they do you can fetch the data with a WDC(Web Data Connector). I have included several resources for using the Tableau WDC below. The first link is to the Tableau Online Help on the WDC feature. Additionally, I included links to the specific section in the community and the Tableau GitHub section on the WDC.
Hope this helps!