I agree RV. I don’t know if you can even rent a passenger van for 9 to drive across country? I am pretty sure Budget rental cats has 12 passenger vehicles in some locations. Some of the other companies might also. I would call Budget and a few others and see what they say.
It would be easier to have two vehicles probably. Then you could have two large cars/vans/SUV’s and be more comfortable.
I have heard of renting buses for a day or week of various sizes, but I don’t know if there are nationwide companies where you can drop of in another state. People use the service when there is a wedding or many people visiting form out of town. They can be rented with a driver or drive yourself, I don’t know the details of that though. I thought I would throw the idea out there because it might help your brainstorming,
Why are you driving? Are there stops along the way you want to see, and do the tourist thing?
Remember that there will probably be a high drop of rate for your rental since it is one way.
Have you considered taking a bus or train instead if your goal is to tour some cities on the way? Bus as low as $219, train as low as $332 connecting in Chicago. Train is nice because you see the country, can walk around, go to the dining car. Usually have a movie. You can spend more and get a sleeping room. You can break the trip into stops in several cities if you want. Probably will cost more that way.