Hello, time for an update with happy news!
First, we replaced the Univolt which has been great as there is absolutely no more hum! Unfortunately, it had no effect on A/C and water heater.
But, am happy to inform you that I am currently listening to the reassuring sound of cold air blowing from my 15K Dometic Penguin with heat pump. We found a super sharp technician in the Austin area who diagnosed it pretty quickly. Turns out there was a short in the cable that monitors the outside temp. Since we will never use, we disconnected it and, Voila, after months of anxiety it is performing beautifully!
As to the water heater, the tech confirmed that the circuit boards use cheap connectors and the connector needs to be replaced. He will come back sometime soon to fix it.
Hope anyone struggling with this finds this thread before they pull out all their hair and melt into a puddle of primordial goo in a hotbox airstream sitting in the Texas sun