I know you have decided to not make the trip, but I hope it isn't because you were scared off by the horror stories. Every woman is different. When I was 34 weeks pregnant with my son, I drove myself ten hours from Tennessee to Florida! If you are feeling fine, and your doctor says it is okay, you should go. You will enjoy seeing your family and they will love seeing you too! Keep in mind that after that baby comes, that 18 hour drive will be nearly impossible for awhile.
If you could break up the trip into two nine-hour days, you would be even better off. But even if you have to do the straight through drive, just follow the suggestions of drinking a lot of water, eating healthy foods, bringing pillows to stay comfortable, being prepared in case the car has a problem, bringing your medical records, and researching an OB in the area you will be visiting. By the way, despite what you see on TV, even if you did go into labor (which I seriously doubt would happen due to a long ride), you would likely have PLENTY of time to find a good place to have that baby. You aren't going to have her on the side of the road!
It sounds like you already made your decision, but if you truly want to go, you should!!!