I had one set up in my Jeep so that it would charge the deep cycle marine battery for my fishing pontoon's electric motor... way, way faster than anything you plug into the wall.
This is what I have in my vanagon. I run the radio, fridge, interior lights and some 12v / USB outlets.
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.
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.
https://en.wikipedia.org/wiki/Battery_isolator
https://www.amazon.com/NOCO-IGD140HP-140-Battery-Isolator/dp...