Ammeter with Auxiliary Battery

[rpmerf]

1970 Chevrolet C20 Custom Camper.

Please excuse the rambling.

Factory dual battery truck, but it has been changed a bit over the years. I have had the truck running in single battery mode for the past 8 years or so. I can't remember what I changed when I removed the second battery in the bed, but I know I took out the factory dual battery harness as it is in a box in my garage. I recently installed a second battery tray and got it running in dual battery mode again. I am using a battery isolator rather than the factory relay setup.

As it was, I had one side of the ammeter running to the junction next to the passenger's side battery. The other side of the ammeter is connected to the junction on the driver's side fender. This is fine with the single battery, but with the dual batteries, it creates a link thorough which the 'starting' battery can discharge into the 'accessory' battery, unless there is some disconnect I didn't notice. It seems the equivalent connection would be be in parallel with a 1 foot long 10 gauge wire between the driver's side battery and the driver's side junction. I don't know that would be effective with such a large, short shunt wire.

How was it wired from the factory? Thoughts? Opinions? Thank you in advance.