I recently just returned from a wedding about 11 hours drive away. I took all three kids (youngest being 5wks at the time) and since DH couldn't come it was all on me! I split the journey going up to the wedding and drove straight through coming home. Like you I am breastfeeding so I had planned stops along the way.
DD slept most of the time in the car and when we did stop I made sure to let her have a good stretch and kick around on her mat on the floor of the cafe. I also made sure that I stuck to her nightly routine so that was why we stopped on the first part of the journey.
Since DD was only 5wks, most of my family hadn't actually met her and everyone wanted a cuddle. I had to watch that she wasn't passed around too much, especially the day before we were heading home as that would make her over tired. She handled the trip really well, only getting grizzly in the last few hours of driving on our way home - by that time we had all had enough though! The only other time she got remotely grizzly was when she got really hot in the car - not an issue if you have air con (which we don't). Luckily we were stopping any way and she got taken out of her carseat etc. I'm sure you'll be fine, hope you enjoy your holiday