I will repost this diagram from before the crash. It is from tests done in the VW wind tunnel - a good few years ago, judging by the model of VW Passat used.
The first case has a Cd of 0.53 but at 50mph produces 76lb less hitch weight
than when it was stationary - this might be 1/3rd of the typically small European hitch weight
In case 2, with a roof spoiler, the Cd drops to 0.45 (15% less drag), but still produces a 71lb reduction in hitch weight
Case 3, with a sloping trailer front and no spoiler, has the same 0.53 drag coefficient as the first case, but has no loss of hitch weight at speed.
So VW reckon spoilers work.