I wonder if you take your vehicle and plug it's numbers into the
Aerodynamic drag and rolling resistance calculator to see if you can get an idea of what's all going on with your current aero, rolling and efficiency.
Then take the same aero numbers and just add more weight to the equation, the weight of the trailer. That would give you a guess at how much more fuel you'd be using with just the weight increase alone.
Maybe someone else could chime in but there could be the possibility of improving the coefficient of drag, or you could hurt it. You're right for trying to keep the frontal area the same and then tapering back.