Driving from Florida to Vermont with 500lbs+ of cargo in my Insight and pulling my trailer trailer with ~1000lbs in it, I averaged 57mpg driving through mountains, aiming for 60mph. I'm certain the trailer was not a net gain in aerodynamics, even though I cleaned it up pretty well. An empty car without the trailer probably could have achieved between 80 and 90mpg at those speeds.
Last fall when I picked up my tires, I averaged ~90mpg driving to get them (~50 miles) and got 54mpg on the way back, with speeds between 40 and 50mph.
So, really, doubling the weight of the car and pulling a relatively aero-clean trailer (which almost certainly still adds some drag) is worth a good bit less in terms of fuel economy loss than leaving the hatch open in the back.