Just an FYI, I have had a few bulldogs that get car sick. None of my other dogs (hunting breeds) have ever been that way. So, if your dog starts acting funny and gets a little frothy around his mouth you may want to pull over and let him run around a bit. Benedryl also helps for N/V.
Whenever I travel with my dogs, I feed them light the night before and morning of travel (approximately half what I normally feed). Give them plenty of water though. Then when you take a pit stop, give em a doggy biscuit or two. That usually will stimulate their gut enough to get them to relieve themself at the rest stop instead of five minutes after you've left the rest stop