Converter and Battery Isolation Switch

I would take the batteries down to your local tire/auto parts house and have them load tested (it's free). Even a bad battery will often give you a decent voltage reading right after you disconnect from converter/charger.

If the light fixtures, ground wires, batteries etc all check out then I suspect your converter might be failing. Not expensive to replace - some/many consider Progressive Dynamics to be better converter - easy DIY install.