
Having lived with a breed not known to be good at car travel, I'd suggest your dog may be more upset with SHOWING (even if you think he does perform well) than getting to the show!! If he hated the travelling he's show you by vomiting, for starters. I'd be very careful about what you give him if you want him to show well - Rescue Remedy is about as far as I'd go with him but again, unless he's vomiting or otherwise upset in the car/van, I'd look to this having nothing to do with the journey!
Might he be picking up your vibes re going showing?
I had a hound who was terrible in the car until, picking up a grass seed which took forever to point, he had to go to the vet all the time. Once this was resolved, he LOVED being in the car. That was the one positive about that time. So keep going out in the car - not necessarily only to a show.