I'm in the middle of potty training, too. People say to never put a diaper on again once you start, but I don't see how. We put on a diaper at nap time and bed time. And when we leave the house. He's just not to the point that he can hold it for very long. He's not ready to go out, even though he does fine at home. and he doesn't have the control to hold it while he's sleeping. And Pull-ups are a joke. They don't feel like real underwear, they feel like a diaper. My son says they're diapers, and doesn't care if he pees on it. So why pay more for them?
So if that's what your son is doing, meaning that he has a diaper on when you put him down for a nap and bedtime, tell him that you're going to put a diaper on him while he's sleeping, and when he wakes up you'll take it off and he can go in the potty seat again. Because right now, I think he's pulling your chain and stalling bedtime. Make it clear that once the diaper goes on, potty training is over for the day (or you're taking a break for nap time) so he doesn't get any more trips to the toilet.
I especially think he's pulling your chain because he gets away with it more times with Dad than he does with you. He's testing you, and it looks like Dad is a little softer.
As for the trip, I totally understand where you're coming from. I was so nervous on our first potty-trained trip! It was Christmas time, too. I considered putting a diaper back on my son, who was nearly three by then. I didn't do it, though. Before hand, we'd practiced holding it. I gradually didn't rush to take him to the potty when he said he had to. I'd finish what I was doing and then take him. He got so he knew how bad it was, and how to hold it. We did stop on the side of the road a few times. We have a little potty seat, but we just let him pee on a bush. That's an advantage of having boys! I also didn't give him much to drink until we got to our destination :)