Well that is cool. Grande Vista is a great resort. We also own there. Looks like the savings you will get from not paying for a hotel will be used up on the rental car. At this point Priceline may be your best option.
Prices are up there, checked
Alamo and it is over $600 for a full size with the best coupon I can find.
Is it possible you could move the trip back one full week and go the following week? Price drops to around $200. Not sure if they could change the week on their reservation at this late stage.