The air conditioner is run via 120VAC, and will have its own circuit breaker.
The fuse in question powers what exactly? Something that was added... Thermostat maybe? Something the previous owner did. Pull the fuse and see what stops working. Unless the wire is small (18ga or smaller), I wouldn't worry about the fuse being a 15A.
The empty slots are usually spares. I added a 12V cig-style plug near the bed in my trailer for a car charger for my phone, and used one of the spare slots and added the appropriate fuse for the wire I pulled (15A).