Wow! We took our first day-long trip and made it. I got one of those handy car adapters for my Medela pump. I pumped every 3 hours sitting in the middle row between two babies while the big sister was in the third row with a third baby. Once I was done pumping, we fed the fresh milk to the babies, then stopped to wash the pumping things/eat/use potty. It worked out pretty well - a 10 hour trip took us only 12 hours.
The babies were a hit with the cousins, even the boys:
Babies with JD, David and Brandon
Nicole feeding one
Nina feeding another
And Sidney after feeding the third (with the babies' big sis)