I've never traveled with a large group of rats, but I have made quite a few interstate trips with 2-4 rats and never had any problems. My girls have all LOVED car rides.
I use a small carrier instead of a full cage, and put something like a sock or pants leg in it -- something they can snuggle up inside of and hide. In my experience, within the first half-hour of the ride, they'll curl up and fall asleep. Along with their regular food, I give them vegetables with a lot of moisture to eat to keep them hydrated (my favorite was jicama, don't know if you can get that outside of the Southwest USA), and whenever we make a pit stop I offer them water (water bottles leak. they need to make a non-leaky waterbottle design!) and let them run around (supervised, of course!) in my back seat. On the really hot trip through Arizona, I also gave them ice cubes to lick.
I always kept their carriers in my floorboard because it was the coolest place in the car (my air conditioner is unreliable at best) and the carriers wouldn't move around too bad.
I've roadtripped with a LOT of rats this way. Athena and Squeakers made a 7 hour drive to my parent's house for Thanksgiving; Athena also made at least two 3-hour drives and a 10-hour drive to Colorado. Velvet & the girls made a few 3-hour drives and the BIG roadtrip to Arizona (about 12 hours), and the boys made that same roadtrip back along with the girls (separate carriers, obviously).
Well-traveled ratties!
The only problems I've ever had with any of them have been with leaky water bottles, and arranging travel arrangements so they don't have to sit in the car at any point while you're eating (because resturaunts don't look too keenly on you bringing 'em in.)
