Richard, let us know if you use an ice maker, and turn it off for a while to rule it out, if you do.

On my bus, similar to yours, there are cooling fans for the electricals down in the bays. Somewhere behind the scenes and out of sight in bay two. They run all the time. Maybe they are going bad.

Have you checked for anything by looking from the outside of the bus, that is, under it or in the bays. Is it louder there or not even heard.

Early on you said you turned off all the breaker switches. I took that to mean those on the interior panel.

Have you tried turning off the inverters themselves after turning off all the circuit breakers?

If you turn off the engine and house batteries at their switches does the sound go away. If so turn them on one at a time and listen for the sound to come back.

I see from your profile that you are in Labor Relations, have you ruled out a bomb?

JIM