I had a foster dog who got sick in the car - every time. I adopted her to a nice man and gave him the heads up about the carsickness. About 2 years later, he and his girlfriend (now wife) had sold the house, bought a truck and FW, and they and the dog were full-timers. Just by being patient, continuing to take her in the car, etc., they were able to overcome her problem. She traveled all over the US and Canada until her death at an old age. So yes it can be overcome.
Watching her and identifying warning signs, triggers, etc. can be very helpful. Other possibilities: keeping her center of gravity as stable as possible can help greatly - have her lay down during the drive; keep her from looking out the windows; keep the fresh air flowing as much as possible (less A/C more open windows); refocusing her attention - give her a toy to chew on.