Hello from Colorado: I've not seen any plans or detailed prints on a mid bath layout for a 71 Sovereign
. I do know some folks have converted their trailers to mid bath because it is a more desirable floor plan.
You will likely develop your own plans based on many, many factors such as waste water tank locations relative to the axles, wheel well interference, window interference, heat ducts, bath vent fan location, and on and on. Once you get your plan and are convinced it will work, then have at it.
The "can of beans" project thread in these section describes how he created a mid bath layout. You might read over his project thread and see what he did. This Airstreamer knows his stuff.
Here is a link to the Airstream, Inc website "document archives" for 1973 model year. You can also research other years and see what you find. It is just a pictorial layout, but it might give you some ideas on how to build one.