Are other similar properties in the area booked in the fall and winter? What activities are possible in the slow times?
Its a problem here in CO, too, but the timing is reversed. Properties are heavily booked during ski season, and somewhat during the fall, but summers tend to be slow. The have built golf courses and heavily promote hiking and biking to try to fill the rooms during the summer. what can you promote that would make people want to visit during the winter?
Most vacation areas have high and low periods. Probably not really too much you can do to change that. You just need to account for that in your valuation calculations and be sure you can get through the slow times.