According to some people, the 2 sides are nearly equal to the 1 top, actually.
If your object-in-motion were the lead object, that might be different, but since it's a trailer, it's going to have to follow the profile of the object leading it, which means that there will be a fairly large compression of fluid (air) above the vehicle/trailer as an assembly.
This is not to say that you shouldn't work the sides, but the top is still a desirable park in which to play, metaphorically speaking.
EDIT - If I really only had the choice to work on one objective, instead of top or sides, I'd simply choose "rear". That'll cover it!
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