Sounds like a dead battery or a bad battery connection. The clicking is probably the starter solenoid. My guess is you need a new battery.
If the vehicle sits for long periods of time (a few weeks, say) without being used, you would be well served to consider getting a small charger for the engine battery to keep it charged. The chassis has a few constant loads that very slowly wear down the battery over time. One such charger that works very well and doesn't need any fiddling once its installed is the "Trik-l-start"; it basically connects the charger/converter for the house battery (when the motorhome is plugged in to shore power) to the chassis battery to charge it when needed. Alternately, you can disconnect the engine battery if the vehicle will be sitting for some time by removing the negative lead from the battery to avoid running it down while parked.
5 years from a car/truck starting battery is not atypical life.