I don't think one thing is the cause of the poor mileage but many. The reason I mentioned the alignment was you had a side impact and the car was not repaired well. As for the effect of misalignment think of a shopping car with the wheel stuck at an angle. Harder to push than when working correctly. Push it and it slows to a stop more quickly.
Pulling to one side is not always alignment it can also be a brake or tire problem. You said your parents are mechanics have you mentioned the pulling and poor mileage to them?
Try slowing down by 5 MPH and check your mileage and then by 10 and recheck it. Most likely the best mileage would be in the 45 to 50 MPH range (my guess) but way to slow for the interstate. This is why there are post recommending instantaneous MPG gauges so you know exactly where your best economy is and were it is best near highway speed.
Check the tire pressure, slow down a little, start a fuel log and find out why its pulling to one side. Make a plan.
