I read through your photo album with great interest and admiration! I'm really interested in getting some kind of AC *sensibly* installed in my 13.5. I have wondered about putting one up front under the dinette, and have also wondered about putting one in the rear venting out the back.
The spare tire is in the way, so that would need to be reconfigured. In either case I have the wood floor over the
fiberglass pan, so drainage would be a bit more complicated than what you set up... I think?
And another option might be, as others have done, removing the 3-way which is not that useful to me... and installing one there. The outside vents are already available...