Taking cats on a road trip?

My husband and I are traveling to my parents' home this Christmas for a week or so, and the problem has come up that we may not have someone available to check in on our 2 cats while we're away, which leads us to think that we'll be taking them with us. We're still looking for someone to watch them, but if all else fails, we're going to have to take them. My parents have 2 adult cats and 2 dogs, and we were already planning on taking our dogs with us, so we can already imagine that it's going to be a clusterfuck for the first few days.

It's a 10 hour trip, and I have read about keeping a litter box available in the car along with food/water to keep them comfortable during the trip, but does anyone have any advice/experience with introducing 2 adult cats to 2 new cats for a temporary period? Has anyone had any luck doing this?