How about adding a bump on top of the roof to increase the attached airflow?
It would add a little bit of frontal area, but with the body being so long, I think that you could really stand to benefit, in the end increasing the CdA. This is is where I got this idea:
TheSamba.com :: View topic - What a drag
And what I get away with it is the arching camper top in the picture increases the frontal area by 17% yet only uses .05% more fuel at 60mpg, and the shape is anything but ideal.
This would be the shape that you want:
