Calbre accepts RTF as an input format (and also offers it as an output format). The downside is that you have to handle your fic conversion in batches, and it takes forever and a day, as you wait for the files to be imported into Calibre - then wait for them to convert - then wait for the MOBI files to save to your destination folder. (And you may find yourself tinkering with the metadata so the files will be labeled in your preferred format and sorting as you want them in your e-book reading program. Which adds another step slowing the process.) Still, at least it makes it possible to turn RTF (or HTML, my usual format when saving fic) into MOBI files, and I tend to not bother firing up the program until I've got a lot of fic to copy to my reader at once.