Hi kurzer,
To the extent I grocked your posts I think you established an envelope of measurement error. That is practical and useful for assessing results.
About your rig, in side profile the leading edge of the roof does not look terrible. I would not expect huge splash effect of bow wave. Thus, not huge benefit from a roof deflector. But I am also made considerable mental leaps based on one pic.
Pix of the actual rig might help us understand your application better. I am particularly interested on top view profile. You have one top but two sides.
If you have not already done so, look up Hucho's compilation of research by others on vehicle aero. The section of caravans is particularly good!
Cheers
Bob