Here’s the troubleshooting steps I would do in this order.
1. Fully charge the battery.
2. Take a battery voltage reading.
3. Remove the negative cable from the battery And let it sit for 24 hours.
4. Take a second battery voltage reading. If it is not the same as #2 you have a bad battery.
5. If it is the same as #2 then replace the battery cable and unplug the connector on the key switch. Let sit for 24 hours.
6. Take another battery reading. If it is not the same, you have a problem in the wiring between the battery and switch.
7. If it is the same you have a problem after the switch.
Please provide us with the units model and serial numbers off the ID tag. That way we can find a schematic drawing of the electrical system and offer suggestions after we know your results.