We generally book about 6 months out, depending on when we are traveling.
If you know your travel dates, I would go ahead and book now. There is a "tier" system to their seating charges. When the first group of seating is sold at the minimum price, it goes up for the next tier, and so on. If you call AMTRAK and book over the phone, they will be able to e-mail you a voucher to use to pick your tickets up at either a nearby AMTRAK station or at the AutoTrain station the day you travel. As long as they do not actually issue the "hard" ticket(s), there is no penalty for cancelling or moving your dates around. Once the ticket is printed, there is a charge for changes or cancellations. I generally pick up my actual tickets about 2 weeks before travel.
I also believe there is a limited number of spaces for SUV's.
Good luck and have a wonderful trip.