editor.addButton('MobileToggleArea', {
      text: '<M>',
      icon: false,
      onclick: function (){
          editor.selection.setContent('<div class="mobile-hide">' + editor.selection.getContent({format: 'raw'}) + '</div><div class="mobil-mehr">mehr</div>');
      }
  });

<p>test</p>
<p>test</p>

<p>&nbsp;</p>
<div class="mobile-hide">
<p>test</p>
<p>test</p>
<div class="mobil-mehr">mehr</div>
<p>&nbsp;</p>

<div class="mobile-hide">test</div>
<div class="mobil-mehr">mehr</div>