How to make the move easier?

We are moving from south Utah to Seattle, WA. It is a 15 hour drive from where we live and I am worried about the stress on the cats. We will not be stopping to stay overnight anywhere, however we will be stopping to eat, bathroom breaks etc. Does anyone have any tips for me? Such as litter boxes? I cannot expect them to hold it that long. How to keep stress down? I have never moved this far with cats before.

I don't have any personal experience, but here is an article about traveling with cats and another one here that might help you.
There are some threads that may help you. Here is another one.
Do be careful about giving your cat drugs. Some have the opposite effect on a cat, i.e., instead of calming the cat, the cat becomes hyper.
Good luck!