I agree that configuring an RSS feed is fairly difficult. Some of the issues are related to our having moved the RSS feed code out of the core Discourse code into a plugin. There are not many sites that I know of that are using Discourse’s RSS feed functionality.
To test things out, I configured the feed at https://feeds.bbci.co.uk/health/rss.xml on my site. Here is how that setup looks:
When I first set that up, all feed topics were automatically published to my Uncategorized category. To fix that, I visited a couple of the topics that had been created to look at this section of the post:
What that is telling me is that the URL of the post is at http://www.bbc.co.uk/news/uk-politics-21668349#sa-ns_mchannel=rss&ns_source=PublicRSS20-sa. The domain of the post is www.bbc.co.uk. I then added this domain as an Allowed Host on my Embedding page. I set the path /news/.* to publish to my “fun” category:
I then deleted the first batch of topics that were created by the feed. Discourse is pulling them in again. They are now being published to the correct category.