Depends on where you go, for a long road trip like this, I will try to stick with major rental companies like Hertz/National/Enterprise/Avis because it will be much easier to exchange vehicle across all states.

If you want to maximize your chances on getting a large SUV with ACC, you can first book with Hertz and National (status match to Executive Elite from Hertz PC) so you will have access to both aisles where you can pick your own car (President circle zone/Executive Aisle). Then you may want to book the specific class you need (large SUV) as a back up.

I'm not sure about ACC on X5, but it's not that hard to find a one with ACC (Like Grand Cherokee limited, Mazda/Subaru/Toyota SUVs), although those are less luxury...

Don't prepay all reservations and just remember to cancel the ones you don't need after picking up.