Aerodynamics are huge. As long as the trailer fits in the car's wind shadow and you streamline it, added drag will be minimal. Here are some sketches I made when designing a trailer for my Insight:
My trailer needed to be 40" wide so as not to poke out into the wind and add very significant drag. Your car is wider so I bet you could get away with a 48" wide trailer. I'd say if it's well designed, you might see a ~15-25% decrease in range. If it has sharp edges or creates a lot of turbulence, you could see half of your range disappear.