On my 2019 Ford C the charge isolator relay is under the coolant overflow bottle. It has a delay built in so the engine has to be running for about 20 seconds before it ties the chassis battery and house bank together. The manual explains it needs to see charging voltage >13.6V on the engine side which starts the 20s timer. Any time voltage on the engine side drops below 13.6 it will untie the batteries and wait for 13.6 again to restart the timer. You can hear this click from the driver's seat after starting the engine. Emergency start switch bypasses this delay.
