Gand Design Momentum 350M Battery Question

Hello everyone! Just purchased a new 2015 Momentum 350M. Although I do have some RV experience I am completely new to Toy Haulers. According to our sales person our new 350M has 1 12 V interstate battery. Our sales person recommened upgrading to 4 6 volt interstate batteries. They offered to do this for us at a price of $900.00. This includes all 4 batteries, boxes, cables and labor. Is this a fair price?

Thanks in advance for everyones help!!