I'm not sure anyone has ever seen an electrical schematic/drawing from Heartland. And if they gave you one, there's no guarantee that manufacturing placed everything as shown in the diagram.
For info on the residential refrigerator power, consult our owner-written
Residential Refrigerator user guide. There's a block diagram with some photos that'll help.
The water heater relay that provides 120V to the heating element is likely in a junction box on your 2018. The junction box is located on the front inside corner of the water heater. But depending on when your trailer was built, the relay might be found in a junction box behind a panel near your entry door. The ceiling fan relay would also likely be in the junction box near the entry door.
On a 2018, the thermostat wires likely go to the roof and fire a relay there. On earlier production, where there was a large air return in the ceiling, mid-ship, it would be in a gray box above the air return.
You might find our owner-written
Electrical Guide helpful.
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When the neutral opens, voltage on one leg can drop way below 120V and the other leg can go way high toward 240V. It wouldn't be surprising to see 40V and 200V with an open neutral.
40V supplied to an A/C unit can damage the motor. 200V supplied to an A/C unit, fireplace, and anything else with electronic boards, can fry them. Sometimes damage is marginal and the device may fail at a later date.
If all the failing devices are on the same power leg as the Power Converter, I would look for damaged wires/connections. Have you pulled the main circuit breaker panel to inspect for damage on back side? If you have a transfer switch (part of Generator Prep), you could also have damage there. Inspect with power disconnected, including unplugging the Power Converter.