Hi Michelle:
Personally, I think it is much safer to travel with the cats in a kennel. You do not have to worry if the cat will escape from the car with a door open or cause an accident by distracting the driver, etc.
I have only had one long (6 hour) trip with a cat – when I adopted Dusty. I took a medium dog crate and used it for the kennel. I made a litter box out of a shoe box. I covered the shoe box with plastic before putting in the litter. In any event, Dusty did not use the box during the trip.
If you cannot find a larger kennel, you could stop ever few hours to give the cats a chance to use the box, eat, stretch, drink, etc. Again, you must be very careful so the cats do not escape the car. The kitties may be frightened by what is going on and will try to run away and hide. You may want to consider putting a collar and lead on the cats before the car doors are opened (that is what I do with my puppy when traveling).
You may also want to check with AAA or a travel agent to make sure you find pet friendly hotels for the trip. Depending on where you will be, it may be too cold for the cats to stay in the kennel overnight in the car.
Good luck with your move. You will have to tell us more about your cats.
Mark