Gotta get my two cents in here. Recently had the horns fail. Yes, I discovered it at one of those rare moments when it seemed really needed. So first off, checking it daily is a good practice.
The coil in my relay was shorted, thus the air valve was not operating at all. My analysis of the cause was laying items on top of the steering wheel when the coach was parked. Once the air is out on ours for leveling, the horn no longer sounds but the solenoid is wired to the battery side of the ignition switch, thus is turned on or "pulled" anytime that the horn button is pressed. The solenoid is rated for about 20% use, so leaving the horn button pushed for hours, perhaps days at a time will burn it up. The moral is do not lay things on top of the steering wheel!
Bruce