Before you judge this is crazy...
Try testing with your windows or door open for one night. There is a good chance that your rig is tightly sealed and the volume of air that a residential window AC can move is thus pulling air from your refrigerator either through the refrigerator door seals or the vacuum break in the doors. You've eliminated power issues and it occurs with the AC running so it can only mean airflow issues. It can also be pulling air from outside the refrigerator, around the sides and into your camper.
I am a retired HVAC technician and have experienced similar in a mobile home when the furnace blower came on the burner flame would dye and smoke entered the home from the furnace. Window AC's just have a lot of air flow volume.
Let me know if this helps.
