"They don't affect gas mileage more than other factors (A/C, underinflated tires, winter tires, etc.)."
This statment is just not true. ANY roof rack will turn the vehicle into a brick going down the road.
Stuffing a brick through the air at speed requires much more energy (read fuel) than the aero design
of the vehicle alone. So my advice is put up with the trunk rack and put the fuel money you save
into a new bike.
Roof racks of any kind will have a never ending fuel cost penalty to pay everytime you use one.
If worse comes to bad buy a small trailer to transport your bikes & gear on. You'll not only save
money on fuel you'll be able to use the trailer for other jobs.