I am using AutoCAD MEP 2012. Is there a command that will generate an Isometric one line drawing from an already created 3-Dimensional drawing?
There is not a command to generate a isometric drawing from a 3D model. But what you can do is put your model into an isometric view and change your display representation to a display that forces pipe to display as single line.
It does take some manipulation of the display system in order to make it work but a "reasonable" isometric drawing can be created. I like to create container drawings that xref in my model so I can use xclip in order to logically break my isometric drawings up. That way I do not have my complete model as one isometric but several isometric drawings that represent the complete design. The project management system is of benefit when using this method.