In my experience, weight indeed does matter if you have a lot of hills to pull up, or if you are stopping and starting a lot. If you are just driving along at 60 mph, weight matters little. The over-riding factor is the aero drag of the trailer. Rounded corners vs square on the front, that sort of thing. I added Airtab's to the rear of two different car haulers, and in both cases I got 15% better fuel mileage. Now, that was going from 9 mpg to 10.5 mpg, so I was still getting lousy fuel mileage, but a consistent 45 - 50 Km's more per tank was welcome.
Brian