I know calibre can grab the RSS feed content to ePub content. But sometimes (most of the time actually) the RSS feed doesn't contain a full text. I also know that there some online service which could convert them to full text feed, but they don't work quite steadily. I wonder if there some Calibre plug-in for grabbing full text from a feed?
In my impression, the main problem is that RSS is usually valid RSS, and the external full text isn't usually valid XHTML, so only browsers with complex parsing engines are able to make sense out of the latter. As most of web documents (websites, HTML) are seriously broken, it's quite hard to read them with a simple implementation. If you have a specific example, one might look into what the problem actually is.