Sounds like your kids are very young. When mine were under 2-yrs it was a little harder to keep them content, but lots of bottles, light music, rattles and toys, and plenty of stops for stretching and "mommy time" were key. It's a definite exercise in patience though...and builds character
Now that they are a little older (under 8-yrs) a portable DVD player and movies is a must. Also my daughter is now old enough and has a Nintendo DSi which keeps her occupied too.
At both age ranges we traveled cross country by SUV/Truck quite a bit (FL, IL, OH, PA, VA, NY). Typical Grammy/Poppy trip is 4-hrs and been doing that for years.
Good luck and safe travels.