Traveling with SEVEN!

My husband and I will be traveling from Texas to Disney in Florida with our 5 boys. I have been looking into rental vehicles to make our trip more comfortable. They are pretty expensive for 9 days. Even a mini-van that seats 7 will not leave much room for our luggage but we can manage. We have rented a home about 15 minutes from Disney so transportation during our stay is a must. Do you have any "insider" knowledge about getting the best rate on a rental car? Are there any places to rent larger vehicles such as a 12 passenger van besides Alamo, Enterprise and such? Of course I am looking for the most economical but comfortable 14 hour road trip for 7 people.

P.S. Our kids range from 10 to 15 years old.