Our two old boys (19 & 17) don't travel as well as they use to.
Joxer (17 yo) becomes motion sick just walking the carrier out to the truck. (could be nerves) The vet gave us a pill that we give him 1 hour before leaving. We also restrict their food on travel days so cut down on the urpies. The pill is actually a motion sickness pill for dogs, we use it off label and only a quarter tab.
Imp (19 yo) has to use the litter box more often. I line their carrier with a puppy pad and it helps if we can't find a good place to stop when he tells us it is time for him to go!
A couple of squirts of Feliway in the carrier before they load also seems to help.