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.
Best way to create a iso view from your 3d drawing is to copy & paste it in a different location, then convert it to a solid and flatshot it looking at it from SW isometric form the view cube. Convert to solids is found under the solids tab by right clicking in the ribbon bar and clicking show tabs and selecting solids. Flatshot is a command that you type in to create the isometric view. I am using 2018, but this still should work with 2012.
Create a horizontal section making it about 50% larger in each direction than is required to enclose your model. Go into the properties of this section line, and change "use model extents for height" to no. Now you have a view cube. Look at this from the left and drag the cube much lower than the model and drag the top line much over the model. Rotate this cube 45 degrees in the side view, then 45 degrees in the plan view. This should give you an isometric view.
Actually, from the side view you should rotate the cube 35.26 deg or invers tan of the sine of 45. This will give you a more accurate isometric view. Then 45 deg in the plan view.
I may be misunderstanding the question but what we do is use the layout tabs inserting a titleblock in paperspace then add mviews. That way there can be iso views next to the plan view depending how many viewports you put in your titleblocks. Also, multiple tabs can be added for numerous configurations.