Wonderfully informative responses, freebeard and skyking.
If I'm to understand correctly, what I really need is a compound curve so as to not create vortices at the corners and sharp edges. An ideal solution would probably be to find a wooden boat and flip it upside down for use as a cap (and double as a boat at our destinations). I'm mostly playing right now though, and don't want to buy a boat. I don't have the facilities to bend wood, either.
What I was intending to build was closest to this, really:
^ The nose, anyway. I can't reasonably taper the sides at the rear, at least at the floor, because I need enough room to lay down and sleep, and the total length is only about 7'. As I have the floorplan right now, it's just large enough for a twin (not twin XL) mattress. This is fine, since my wife and I are both small people, at ~5'6" and 5'2". I could reasonably round the roof, I think, but not at the same time as a progressive taper from front to back, and it needs to be tall enough to sit up in. Right now the peak of the roof is 44" above the deck, which puts it at around the same height as the car.
A rough approximation of what it would look like behind the car:
Do you think a level, rounded roof at 44" high, with no front to back taper, would have less drag than a flat roof with hard edges, a peak at 44", tapering to 32" at the back?
EDIT: I should add, it's not a 48" wide deck, but 40".