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‘94/95 Safari Trek 3060, 5.9 version on Oshkosh chassis.
I’m having electrical issue with 12 volt system. When I turn on the rear bedroom ac it knocks out my 12 volt from battery to inverter. And normal 12 volt functions, ac runs fine. When the front living room ac is on not an issue.

Puzzled on why the 120 Ac has anything to do with the 12 volt system.

So, you do have an inverter/charger, not converter. Or is this a separate inverter?

Assume you have verified that 12 VDC is present at the house batteries, but not at the inverter/charger when this happens? Agree, unusual as all you normally have is a high-amp slow burn fuse between the battery and inverter/charger.

Yes, likely not related, but as a safety measure, I would check all connections in 120 VAC breaker box and ATS to verify they are clean and tight.