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by jws 3249 days ago
The devices used in better RVs are a solenoid which combines the batteries but only when the vehicle battery is above a certain voltage, typically set for a mostly full battery with surplus alternator energy available.

This avoids the voltage drop of the diode which confuses the charging situation for the house battery. It is also save the 2-5% power loss and heat dissipation on a diode.

As a bonus, you get a switch to manually engage the solenoid for engine starting when the vehicular battery is weak.