We have 2 cats, one is a tortie mix 16 years old one is a 2.5 y.o. Maine coon 18 lbs. worth. They travel in our truck with us. I set up the back seat by covering the seats with blankets and the floor with a waterproof mattress pad in case of accidents and litter box on one side. I have a double cab Sierra and use a corner triangle shaped litter box that fits fine. We still have to stop and take care of any deposits as quickly as we can.
It takes them both a little while to settle in and get over any anxiety and both have barffed and blew hairballs. They're cats. We tried drugging the Maine Coon and its not a good solution over just letting them adjust. They do finally settle in.
In the trailer their litter box is under the bed in the blanket box storage area. It works and its out of the way eventhough we have had to get up in the middle of the night to take care of those deposits. You do what you have to do for the fur babies. Our cats travel fairly well and settle in fine when we are parked. It is do-able with some effort on our part as their humans.