[bushpilot]

additionally...

kinda sounds like the switch to combine the engine (chassis) & house batteries isn't actually combining them....you sure theres no isolation relay in-between the chassis & house batteries (as is usually the case)?

I'd put a load tester on the house batteries and see what the amperage is...AND then id combine the batteries and test the house batteries again to see if theres and increase in amperage.

Harbor Freight: battery load tester