The build quality of your Kamm Extension is very good, I hate to sound critical, but I believe you have made the taper too "fast". I'm sure it helps, and as we can see from your tuft tests, flow remains attached, however, if you didn't pull it in quite so much, I suspect it would work even better.
The Kamm should maybe be less tapered like the back of this camper concept.
I know it seems counterintuitive that making the back end bigger could be better than the smallish back you have created, but it has to do with keeping the shape of the vehicle close to the Aero Template. Also notice how the taper on my concept camper drawing starts very gradually, the rate of change on the shape is slow. You wanna always try and not make abrupt changes in the air flow on the back end of a vehicle.
As far as the AC fairing goes, I think if you just cut off the back foot of it for air flow, it will make very little difference in the Cd improvement you have made by making it a full fairing. It will still work very well to reduce drag, and it looks very well designed, cutting the back off will allow it to operate as an AC unit without having to futz with it or make up some sort of louver thing.
I only mention all this cause it sounds as if you project is a sort of prototype and you may make another version 2 of the same thing, I'd hope you consider making the Kamm shape less aggressive if you do.
Again, Excellent Job!!