The aero drag wasn't the only thing giving you bad mileage. The extra rolling resistance from the tires of the trailer (did you check the tire pressure?), and the extra weight are probably the bigger factor. Even though it's plywood, the frame is still heavy metal (I hope).
And, I'm guessing that the route you took wasn't a typical route, so you weren't able to plan it as well as you could if you drove it every day.
But aeromodding a trailer would look pretty awesome
