If you hear a click with no engine
rotation it is either starter solenoid, secondary power to the starter or the starter itself.
If you hear a click and the engine appears to be trying to rotate it indicates high resistance, bad wiring
or weak batteries.
You can turn on a cabin
light and watch when the starter is activated. If the light dims noticeably it could indicate high resistance (bad large wiring or loose connection) or a weak battery. Slight dimming could indicate a bad solenoid.
I would check the connections on the batteries and the starter first. Remove clean and retighten all connections.
My guess is the solenoid. The reason for that is you state that often (85% of the time) it workd on the second try. This would indicate the batteries are charged, the wiring is OK and the solenoid is intermittent.