Fyi, rthml not rhtml is.  Because I burned a day troubleshooting and researching, among other things, my version numbers, mod_proxy_html, and relative_path plugin in rails to see why I am getting the message:

No rhtml, rxml, rjs or delegate template found for fleet/app/views 

, I thought it would be appropriate to share the solution.  My render method (render_to_string, actually) is using a :partial call "fleet_import_format" which does properly translate to /app/views/fleet/_fleet_import_format.rhtml, however, my fingers called the file by the wrong extension: _fleet_import_format.rthml.  I accidentally swapped the t and the h, and it made all the world of a difference.  :(

Happy to have found my error.  Embarassed to have made such a flub.  Happy to share in case it helps someone else..!