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 mos

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.