House/motor home battery

The battery disconnects would have a connection to the battery even in the off position. This is necessary to have power to operate the battery disconnect.
OP: Have you located what type of battery charge system you have? Is it a relay operated one or a diode based.

See OP's previous post.

Clicky

What chassis is your Tioga on?

Wait a minute please.

When batteries are switched OFF a mechanical switch is thrown and a spark would have jump a quarter inch to complete the circuit.

The VEHICLE battery circuit needs troubleshooting with meter and test light. Are you ready to do this?

this a Cab light ?
maybe the light is connected to the house batteries, instead of the engine battery

let us know if the battery goes down while disconnected. that means a bad battery

When I come back after a month or so I am able to jump the engine from the house battery. I have a switch in the cab that lets me do that. This is the first time I tried un-hooking the neg side when the battery was fully charged.

If I understand you correctly, you have disconnected the negative of the engine battery and have turned the house battery disconnect switch to disconnect. You still have power to something in the cab.

Seems obvious that the battery disconnect switch does not completely disconnect the house battery.

Are you asking why the engine battery is going dead after a month? If so, have you measured the current drain on the engine battery?

You could have a problem with a diode based battery isolator allowing power to go through it. (Bad diode)
Many of the diode are contained in a blue housing.
Look to see if you have a switch that is marked "dual". This could energize a charge relay. If relay is energized power is connected between the battery systems.