I'm most worried about the driving "straight-through" part. In case of an accident it'd be best to have them all in crates.
[caveat: I drove across country in '04 in a Miata, my Samoyed riding shotgun -- but I took 6 weeks to do it]
At the least, definately have the cats in a crate so they don't freak out and get under your brake pedal. I have one of those net barriers and it would not keep a cat out of the front. It barely keeps a determined big dog in the back.
Plan on stopping every 2-3 hours (3 being the absolute max) and letting your pup stretch her legs.
WATER! Keep extra water in the car (at least a gallon -- what if the E breaks down?). If the trip is stressful on them then they will pant more and need extra hydration.
Travelling in summer you know you'll have to be so, so careful. Good that you're travelling with a friend so you can take your breaks but the animals can be watched and have the a/c on or windows open so the E doesn't turn into an oven.
VET RECORDS: Get a copy of your vet's records on all your animals. Especially carry proof of rabies vaccinations.
PET PHOTOS: In case one of them gets away from you and is lost, make sure you have extra photos that can be used to identify them. In digital media and hard copy pics.
PET ID: Make sure they all have current ID tags. When you're travelling that means a cell phone number that you can be reached at while you're on the road. Microchips would be an excellent idea, in addition to tags.
KEEP ANY ANIMAL NOT IN A CRATE, ON A LEASH WHILE DRIVING: So that you have a better chance of grabbing them if they dart out of the car when you open a door, keep a leash on them while driving. Unless they're in a crate.
CRATE FAN: battery-operated crate fans hook to the cage door, provide good ventilation and don't cost much. Worthy investment.
LITTER BOX: I almost forgot to attempt an answer to one of your chief concerns. I can't imagine driving with a litter box in the car. Yuck. Have your cats ever been on a leash? You've got time to get them used to it. A lot of people walk their cats. Otherwise, I suppose you'd have to go with a litter box. Poor you. pee-yoo!
GOOD LUCK! You are smart to plan so far ahead and thoughtfully.