M.F.
At that age, sometimes it's fun to just hop in the tub with your little one. I'd sometimes grab a quick shower first, then fill up the tub and have my son or daughter come in. He can lean up against you and play, and it's a good way to introduce the "big tub" in a non-threatening way. The other suggestion about putting the baby tub inside the big tub helps with the mess, and also introduces the big tub. We had to move both of my chubby babies out of their baby tubs before they could completely sit up on their own, so we put a bath towel down in the tub, and just held them up the whole time. A little more difficult, but it wasn't long before they'd sit on their own.
As for car travel, if it's possible to head out late afternoon/early evening, drive for an hour or two, stop for dinner, then let your son doze off a little while after dinner, and get to Gatlinburg late, that might be an option. Or, gently get him up early and get a few hours of driving in before he's fully awake in the morning. Luckily, 6-month-olds still sleep a fair amount, and if you can time driving around sleeping, it's not too bad. Believe it or not, it's actually a little easier to travel at that age than older...I'm a little concerned about my super-active little guy who'll be 1 1/2 at our vacation this year! Don't be in a hurry, and just follow his cues. Even a walk around a mall or Wal Mart might be an enjoyable change of scenery for him. Enjoy Gatlinburg--we love the Smoky Mountains area. Check in the visitor's center for handicapped-accessible trails...they're stroller accessible too. :-) At that age, you can also take along a snugli for hikes. The weather should be great when you go...I'm jealous! Enjoy, and good luck!