The advantage of the RV roof air conditioner is:
a] simple to mount & install
b] centered over axles
c] easy to service
d]
great air distribution
The last is the critical piece to address. The cost, complexity, etc of other types keeps them relegated to small trailers, overall due to cubic volume and distances.
That is not enough to overcome what is not that great an aero penalty.
This comes up on RV forums. Look up
Smokeless Joe on
Airstream Trailer & Motorhome Owners Community for a custom installation that will be of interest to you.
Here's an introductory thread:
Ducted AC?- Crazy idea? - Airstream Forums