In Tinymce editor, how to set default font family style in select list?
You can try execCommand to make that work. 


  Executes a specific command on the currently active editor.


Try this.

editor.execCommand("fontName", false, "Arial");


Another way is to use the content_css configuration option and name an own css file where you can overwrite the editors defaults (the content.css you were recently looking for). In your init function use something like

content_css: "http://sitename/css/my_tiny_styles.css",


and in my_tiny_styles.css or whatever file you choose you use

font-family: myfont1, myfont2, sans-serif;