We have been taking 1 to 2, 10 hour trips and one 15 hour trip in the car with my kids, every year since each was 6 months old. We travel from central Illinois to Nashville, Tennessee and to Salem, West Virginia to see family on my Husband's side - His dad and step mom are in TN, and all his aunts and uncles and cousins on his dads side are in WV.
Usually there will be one solid hour of whining and crying near the end of the trip. Other than that they have always been very good.
We either buy a few cheap new dollar store toys or set aside some toys for a few weeks ahead of time ( hide them so they don't see/play with them) and have those for the car ride. We have little DVD players, lots of books, lots of snacks - when they were littler, Gerber Puffs, and those freeze dried fruits, and freeze dried yogurt drops, now goldfish, teddy grahams, apples, bananas, raisins. Special drinks and snacks and they don't get very often at home. We have kids CDs of sing along songs, and CDs we use at home for bedtime and that is the "cue" that it is naptime. we make sure to take blankets and pillows from home for better sleep away, and of course their favorite security item - blankie, stuffed animal, lovey.
Magna doodle, aqua doodle, paper and crayons ( just make sure to put the crayons in a ziplock, in the cooler with the food, so if you forget them in the car, they wil not melt all over if they are in the sun). Also, any magnetic things - we found a little bunny that is like a magnetic paper doll - you can dress him up in all different outfits - we just found an old cookie sheet to stick the magnets too - that was lots of fun for my daughter at that age.
Make sure to take long breaks at rest stops and get out and RUN around - maybe take a ball or frisbee for her to chase after ( I know it sounds like I am talking about a dog here, but you need to get the wiggles out when you do take a break)!
It is do-able!
Good Luck!
Jessie