Check to make sure both of the lines coming in are putting out the proper voltage. In a two year period I replaced the ATS in my fifth wheel thinking one side was going bad as one would either disconnect, chatter, or buzz. Determined the root cause was a loose neutral at the utility pole causing the voltage on one leg to drop to 107 V. Power company fixed it. After getting a new ATS under warranty, I opted to instead install a Blue Sea Systems manual transfer switch. I don't have a need to ensure 100% always available power, however, I did install a pure sine wave inverter with a transfer switch so at the very least, certain devices in the bedroom can seamlessly switch between shore/Gen or battery.
That's been my experience. Sure, an ATS is nice, but in my case, it seemed more problematic than it was worth. I think it was rated to transfer down to 100 volts but never could hold it.
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