99 Club Car battery issue

[hookdaddy]

I seem to have something causing the last battery in line before the solenoid and the motor to run down prematurely. I've replace the OBC and had the charger checked. It seems the batteries all charge and check ok. After charging I load test the batteries and the all show 8.5 volts and drop to maybe 8- 8.25 volts.
My problem is after a lot of use ( maybe 6 miles ) my voltmeter on the cart will show it losing voltage while going down the road and the cart needs to get back to the charger quick or I might not make it home. When this happens I check the batteries again and 5 of them have 8.5 volts and the problem one has 7 volts and load tests poorly. I have replaced this battery but the same drain happens always to the last battery before the motor. I switched the order of the batteries but the same drain happens always to the last battery
Any ideas? What's causing this?