Connecting the house battery and the engine might potentially cause isolators to malfunction. This enables the usage of your RV equipment to drain the batteries in your engine. The higher voltage battery will attempt to charge the lower voltage battery as a result of the two batteries' mismatched voltages, which might affect the lifespan of both batteries. With the engine off, you may check the voltages on the three isolator terminals to determine whether either battery is putting out voltage on the alternator terminal, which ought to be zero.