We took our daughter on her first long trip when she was 4 months old and still breastfed. We drove 1100km, but broke it into 2 days. I had a breast pump to try and express on the trip, but just didn't happen. She slept most of the way thankfully, but arked up in about the last hour of the trips. I sat in the back with her most of the way and played toys with her. We would stop every 2 hours for her to have a bit of a feed, and it seemed to keep her content. We managed to do the trip in 11 hours all up.
2nd trip the car seat was around the other way and she was 9 months old, same distance, and bottle fed. We found this so much easier as she could see mum and dad, and we could make her bottles and feed them to her in the seat, so not as much stopping. This time we did the trip in one day and she probably only whinged for a few hours throughout the day. Lots of toys to keep her entertained and she was pretty happy.
Good luck with your trip, try to break it up a little if you can. How far will you be driving?
DD1 July 2004 DD2 August 2007 DS September 2009