Dual battery discharge problem

[alan1nckc]

Transit Custom minibus yh66lfa. The batteries discharge while the vehicle is parked. According to the manual the starter battery should be protected from this.
It first showed up in January when the vehicle had been parked for 3 months. It needed a jump start, and seemed OK after that. In March when it was next needed, it needed a jump start again, but after being parked overnight again needed a jump start.

When I connected a charger directly to the rear battery, which I think is the one for the starter, both batteries showed the same voltage throughout the charge. The manual seems to say they should not be connected together when not running. This indicates that they are connected together.

After this charge the engine started OK. Two days later it needed a jump start.

Before I knew that there were two batteries, I took the bus to Bedford Batteries, who put a tester on the rear battery and declared it OK and not in need of replacement. In view of the above it might have been measuring both batteries in parallel.

HELP!

I'm not clear where to go from here. While replacing both ten-year-old batteries might be a worthwhile step, if there is an unwanted discharge, it's only going to hide the problem for a while.

[Altransit]

I would suggest that the first step is to ascertain whether the batteries are simply connected in parallel, or are wired through a factory installed dual battery system

Either way, if the batteries are ten years old, then they're probably overdue for retirement.