Has anyone run into the problem of <div class="mktEditable"> not working in responsive landing pages?

You add the class "mktEditable" to make a div in an Email Template editable in the Email Editor. In LPs, this works differently. You should check out our docs for both Free-form and Guided mode landing pages. Each has different functionality and syntax.

Overview: https://docs.marketo.com/display/DOCS/Understanding+Free-form+vs+Guided+Landing+Pages

Guided Templates: https://docs.marketo.com/display/DOCS/Create+a+Guided+Landing+Page+Template.​ In guided templates, you add editable elements and/or variables in order to declare what's editable in the guided lp editor.

Free-form Templates: We don't have a great doc for Free-form landing page templates, but they are made up of any HTML you want (basically the skeleton look-and-feel of the page) with at least one "mktoContent" div where content would be dragged in from the free-form lp editor. You may, optionally, also specify other divs to have the class "mktEditable" which means the CSS properties of that div will be accessible for editing in the "Template Elements" section of the free-form editor.

Hope this helps. Let me know if you have more questions.