[throwanem]

Both no doubt exist. A diode would let the secondary battery charge, but never let the engine draw on it; a solenoid-driven contactor would let the engine both charge and draw on the secondary battery, but only when the engine is running.

Combining both might not be the world's worst idea, depending on the design purpose. The diode alone would allow the secondary system to draw from the primary battery, potentially drawing it down far enough that it couldn't turn over the engine. The contactor would prevent that by completely isolating the two circuits while the engine is off; the diode would prevent the secondary battery being drawn down while the engine is on. If you need to draw from the secondary to turn the engine over or draw from the primary into your hotel load, you can short across the diode-contactor pair with one side of a set of jumper cables.

[jws]

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.