I setup RSS feeds about 2 months ago and items marked as read and have not gone away eventhough I have it in the settings set that they should have been gone weeks ago.

I have the same problem with arxiv.org, and I checked: the items are removed from the feed. Now, I noticed the following strange thing.

I configured my general zotero settings for feeds as follows:
* Remove read feed items after: 3 days
* Remove unread feed items after: 30 days

Now I added a new feed and looked at the "advanced options" (without modifying them):
* Remove read feed items after: 30 days
* Remove unread feed items after: 3 days

It has swapped the settings for read and unread. Suspicious.