Have not tried the Sleepypod myself, you'll have to let us know how the dogs tolerate them for that long drive! I have used car harnesses on our dogs, a crate doesn't fit in either of our cars, and not ones for both dogs. The main issue I've had with car harnesses is the dogs wanting to stand up and change positions and getting tangled in their own or each other's harnesses or seatbelt straps. Lark likes to lie down and take up most of the room in the back seat, Chase prefers to lie on the floor if we're in a sedan. They normally ride in the back section of my midsized SUV and I don't think the sleepypods would work back there.
Aside from the harness issue, my travel advice is to take copies of their vaccination records and look ahead to see what vets might be available at your destination. If you're visiting family, maybe they have a vet they use. I just like to have some idea of what is available in case of illness or injury at our destination. Make sure they have ID tags on their collars and that they're wearing their collars in the car.
My dogs don't really do anything in the car except sleep on long trips. I take them on 6-8 hour trips pretty regularly and aside from a stop midway to go to the bathroom, there's not much else that happens from their perspective. Packing a car for a long trip can make things crowded so I try to keep a few things within easy reach. I keep bottled water and bowls handy for a drink when we stop for a break. I keep their leashes where I can get to them readily. When we stop, it's also often at a mealtime for me, so they might get a plain cheeseburger or chicken sandwich from whichever fast food place I stop at, or else I keep a snack handy for them to eat while I eat. I figure a plain McDonald's cheeseburger every other month or so is not going to cause a health issue for them and once the car smells like french fries? How can you just give them a handful of kibble?
I have a bag of dog stuff that I pretty much take on all our trips, it never gets unpacked. I keep extra leashes and collars, a few first aid supplies including hydrogen peroxide, since one of my dogs used to eat socks regularly, a few other things I almost never need but feel better having.
That's a long trip but if they're good in the car in general, you probably won't have any trouble. Have fun!