Hello S.,
We're about to be in the same predicament. Our son just turned three, and we've got a small trip planned to Colorado. We're in the process of training him this weekend, and our trip is planned for the end of June into the first of July.
What we've got planned is taking his portable chair with us and using plastic pants while we're in the car. Otherwise, we'll just keep reminding him to go while we're at the hotel. We also plan on taking lots of spares of everything! (shorts, jeans, underwear, plastic pants) We may even make some time to go to a laundromat!!
As for the nighttime, this may sound harsh, but we wake our son up before we go to bed so he can go potty. He's really good at it. We get him up, take him to the potty, he does his thing, washes his hands, get him dressed and he goes back to bed wonderfully. He wakes up dry the next morning.
About your question concerning the pull-ups; we don't use them. We simply use the plastic pants over his underwear, and it works beautifully. Of course you can use what you want, it's your choice.
Hopefully both our trips will be successfully dry!!
ls