I'd suggest putting kitty in a big carrier with a small cat box. That way, everytime you stop and open the door, you won't have a cat trying to escape (my folks lost a cat for good like that on the Alcan).

They make small litter boxes to go in carriers. If you can't find one, I guess you could use a shoebox with a trashbag liner to catch the "liquid". Then you can dump it at night.

By the way ~ Welcome to Pet Talk! I hope you will stick around and let us know how your trip went.