I do the same as DanL, pack a cooler of about a week's worth and they shop when I get to my destination, except when I'm flying, then I just shop when we get there.
Funny story though (and a bit of a warning). Don't put the cooler behind the driver's side seat! I ended up getting into a wreck on one of my trips, flipped my car twice (luckily no one was hurt, and everyone was wearing seatbelts, including the dogs) and the lid to the cooler flew off during one of the flips. The ice had melted and I got showered in a lovely raw chicken scented water.... That was nice....