We also take a lot of road trips and if we were driving through the night, we'd have them use pull-ups, but not during the day. Although, I have to strongly encourage an aletrnative like others suggested. We used to have a "blow-up" potty chair, but it must have sprung a leak, because we can't find it. Anyway, we now use a bucket. A plain 'ol sandbox bucket. A large coffee can would work, too, because you have a lid for it. We found that even if we try to have everyone pee when we stop, inevitabley, there was one of the kids who went, but still had to go half hour later. Line the bucket with a grocery sack (use at least 2 if you go with those), or a re-sealable bag. There wasn't always a place to stop when the urge arises- especially if you are traveling on Christmas day. Make sure you have plenty of wipes and/or tissue and sanitizer.
*Funny story I had to share...among many, actually-LOL! However, this one is appropriate for a public forum! :) A friend of ours always made fun of us for our bucket use when it came to trips. Last May, he took our 7 year old and 9 year old back to Iowa with him to visit his family (he works in the Columbus area and went home for Memorial Day weekend). We asked him if he wanted to take the bucket. He laughed at us and said no. However, as the trip progressed, he found out why we do that. The kids ate McDonalds and with that meal, had pop. It goes through them really fast and often, apparently. He had them potty when he stopped, but didn't realize he'd be stopping a whole lot more often with 2 kids and small bladders! He then apologized to us in a phone converstaion and said he'd never make fun of us again!!!
Good luck with your trip!