Currently, the font family will not be passed to the Editor content. It's reproducible when loading tools from xml file. Possible workaround is to use ContentAreaCssFile to set such style or on the OnClientLoad event:

    <script>
        function OnClientLoad(sender, args) {
            sender.get_contentArea().style["font-family"] = "Arial, Verdana, Tahoma";
        }
    </script>