When I moved from Providence to Ann Arbor in 2002, I rented a cargo van from U-Haul and had the six carriers of cats on the flat floor on the back. The trip was about 14 hours, which I broke up midway with a stay in a motel in mid-Pennsylvania which I found out about from a "pet-friendly hotels and motels" web site.

As it turned out, the cats did fine staying overnight in their carriers in the van, so I didn't have to try and shift all six carriers (and perhaps find that the motel wasn't quite THAT pet-friendly ..). Each was in a larger-than-life carrier with a tiny litterbox in it and food and water holders attached to the doors.

I don't exactly recommend this as a way to move them, because I was a nervous wreck driving that huge monstrosity of a van, but we all got here just fine.