Gail is correct, you are going to have a very hard time finding a conventional rental that will allow you to take a vehicle across-country. They have really tightened up on that, especially with larger vehicles.
Here's something else; have you ever *driven* a 12-passenger van? I have. Those things are tanks, and the suspension is horribly uncomfortable, the shocks are so stiff you bounce on the seats. You'll all have pretty bad backs by the time you arrive.
Have you considered renting an RV? Those are normally allowed to go anywhere, and it is definitely a more comfortable ride than a shuttle-type van. They are not cheap, though. The rate for an RV that will hold that many passengers (not sleeping space, just seat space) is probably going to come in around $1K for 8 days.