Volkswagen tested roof spoilers in its wind tunnel back in the 1980s when they first became popular and found a significant effect.
The top case has a Cd (drag coefficient) of 0.53 and in the second case with the roof spoiler, that drops to 0.45 - a 15% reduction. In round terms you would expect a 15% drag reduction to give a 15% improvement in fuel economy.
If I had to choose the accuracy between an internet discussion and an automobile manufacturer's published wind tunnel data, hmmm, let me see..... I'll pick the wind tunnel.
The third case shown was what has become the standard design of European trailers with a sloping front. This has exactly the same drag as the square-fronted trailer, but does not get a light
at speed - both the other two cases suffer from that.
(PS first posted 2006, I see)