I would only use gravol as a last resort such as if you were more than an hour into the trip and the dogs were still crazy. They will likely settle fairly quickly especially when you get up to highway speeds.
I use seatbelts for the dogs and have since Riley was a puppy. This is the model that I like the best
www.kurgo.com I don't like to connect the dog to the seatbelt as I find connecting them lower down like that can get them tangled. I actually have short super strong leashes that I anchor to the child anchor bolts and bring behind and over the top of the seats and then attach them to the dog's seatbelt. Kurgo also makes an auto zip line to connect the dogs
http://www.kurgo.com/products/azl.html.
I would recommend trying the homeopathic remedy Aconitum nepalis (aconite) 30c or 200c and give one hour before and then again right before travel. It is very good for fearful and/or stressed dogs.
If you get the seatbelts I would recommend a few short trips individually before tackling the long trip.
Good luck.