Aerodynamics do make a big difference in power needed (and hence in fuel milage) at speed, as you've seen. Weight, in itself, is comparatively a non-factor for flat highway driving. For stop-and-go driving at low speeds, things tilt more in the opposite direction.
I don't know if it applies in your case, but the back end of the trailer makes at least as much of a difference in air resistance as the front end. A squared-off back introduces a lot of turbulence and air resistance; this is why practically all new cars have sloping backs, even if it makes for poorer back seat headroom or poorer trunk access or poorer rearward visibility.