2020 Grand Design Momentum 320G - 12V electrical problem

[Mike and Mary]

Hello,
I have replaced two 12V batteries (Parallel connection) with two 6V gulf cart batteries (Series connection) as an upgrade. I did this last November and have used the RV several times. The RV is stored indoors and connected to a 50amp service.
I noticed the battery indicator on the control panel appeared to indicate the battery power was decreasing in time, eventually losing lighting if unplugged from shore power. I removed the 10 month old batteries, returned to the supplier for testing. They tested above 90% charge and I reinstalled them in the RV.
Given:
Everything seems to work properly while plugged into shore power or generator is running.
The battery disconnect switch is on and key is locked in the vertical position.
The control panel shows batteries are fully charged, but does not light up when on battery power only (no shore power or generator power).
The pass-through light works, but no other lights work when on battery power only.
I'm not sure if the batteries are charging while connected to shore power. When I initially started problem solving, I connected a external 7.5 amp charger/tender thinking the batteries were discharged. The supplier verified that the batteries are charged and capable of receiving a charge.
I haven't replaced the battery disconnect switch, but think it maybe the problem. Seeking others advice before doing so.
I greatly appreciate any advice/recommendation/suggestions to resolve this problem.
Thank you for your help!
Mike

[el Rojo]

Look for a loose wire. Follow your battery cables to the cut off switch then to your panel in the front bay.

[Corky2]

You can determine if your batteries are charging by monitoring the voltage at the battery with the charger off and then charger on. You should see an increase in voltage (~13-14 volts DC) with the charger on and charging.

[FlyStar]

A volt meter in the tool box will be your friend. I have one of these.

Fluke 107 AC/DC Current Handheld Digital Multimeter by Fluke, Gray https://a.co/d/fY6e4AN