12 available in house ONLY when the motor is running

I opened up my old inverter, the one that I took out of the RV.

It turns out that inside of the old inverter, the BLACK battery negative is connected with a BIG shunt, to the GREEN chassis ground cable lug. For the new inverter, I added an outside cable to connect NEGATIVE to GROUND (12v, not grounding any 115v). (to accomplish the same thing as the shunt insidet he old Trace inverter)

After that, everything seems to be back to how the RV was before.

Thanks a lot guys for keeping me positive and giving some food for thought.

First guess is that you have polarity wrong someplace in that system. Batteries, inverter, ?. Polarity should be the same from chassis and house batteries.

Battery disconnect
Not
Salesman switch
That switch is not for the RV salesman
It is for the owner to use during RV storage

Sounds like a wire was moved or dropped during a new battery swap
If it was the switch aka power relay, engine running should not make any difference

I have done a fair amount of testing and troubleshooting with my multimeter

First of all, I must say that I did replace my old Trace inverter with a newer Freedom SW inverter a week ago. I believe that I connected the wires correctly, however it is possible that something was done in error.

With that background in mind, here are my findings.

1. Apparently, there is a grounding issue.

2. When the engine is off, the negative side of chassis batteries is somehow connected to the positive side of house batteries, through an unknown electrical path with some decent resistance.

3. Near the inverter, there are three thick 12 wires, red, black, and green. I connected red and black to the power input of the inverter and green to the grounding lug on the inverter. There is 12v between black and green.

4. I attempted to connect (with a 12ga jumper wire) the black and green cables. That itself resolved the power issue, in the sense that the house has 12v power, the generator can start, etc. However, there is heavy current going through that jumper cable and I quickly disconnected it.

In my older class a, there were two salesman's switches above the entrance door. Labeled 'Main' and 'Aux'.

there is typically a switch to turn the 12 volt on and off near the front door. Sometimes it can get bumped by persons or dogs when entering or leaving the coach. Ours is labeled "Bat Cut Off"

Matt, I am sorry, what do you mean by salesman switch?

Start by measuring both banks of batteries. Take some pictures for future reference and print them out. Write on the pictures the readings you get. Take the pictures in the lowest resolution possible so you can attach them to an e-mail

Is the "Salesman Switch" off?

Matt