Maintaining RV batteries

You want a battery tender, NOT the RV on board charger as it is likely NOT regulated properly. A battery tender runs somewhat lower voltage than your charging system what they call "float" voltage. That voltage is supposed to minimize battery activity and water loss

I would NOT run a solid state battery isolator. Instead use a high current what is called a "continuous duty" solenoid. This solenoid is activated by the vehicle ignition switch and connects the two systems when the key is on