We also have the storage bag for the 9 inch tv/vcr that mazzarafe mentioned. It is made by Steel Horse and can be bought at several places on line. Target also carries it, but when we were looking for one it was always out of stock. It holds the tv very securely, attached to the seats, and it also unhooks easily and becomes a carrying case.
The tv/vcr in the car is a great way to keep the kids entertained but I wouldn't use it unless it was absolutely secure. Unfortunately, you have to think about where it would fly in the case of an accident. Bungee cords and things like that are not going to do the trick. I agree that the Steel Horse bag is definitely worth the money.
Good luck and have a great trip!!