I’ve had my 83 B250 camper van for about a year and can quite seem to figure this out. My cabin lights and vent fan only seem to turn on when I have the door slightly ajar. No matter if I’m on the main battery, switched to the leisure, or connected to shore power. The previous owner had added a stereo and messed with the wiring up front, but apart from that I have no idea where to try and find the disconnect. It obviously receives power, if the switch associated with the door is triggered (or untriggered) as it may be.