That Chassis Battery is the one that stays up for months without me having to disconnect anything or turn any switches off.
The House Battery Disconnect should not have to be turned OFF to keep the Chassis Battery from running down.
Are you leaving something ON that you didn't notice, dome light, map light...?
I think a problem in the Charging Circuit (which is basically the Alternator since the Regulator is inside it) could drain the battery, but I also think it's too soon to go after that.
Four items that I can think of connect the Chassis Battery to parts of the Coach
1. Running, Tail, Marker Lights. Shouldn't drain Battery unless left ON
2. Stop Lights. Should be OK unless Brake Pedal is Stuck
3. Battery Isolator. Lets Chassis charge House Battery but doesn't let House run Chassis down. Would have to be defective to run Chassis down.
4. Start Assist. A Solenoid with a Dashboard Pushbutton. Allows House to give Chassis a Jump Start. Again, would have to be defective.