if you turn the power off does it stop?
the way your describing it where you turn lights on, it gets quieter which doesn't really make sense for the converter fan as it should be off if you are using now power and only start up when the unit starts getting warm. every rv I have owned if your power usage is low it is silent then as you use more power the fan will start to keep the converter cool. so adding a light should make it louder if it is the converter fan unless something is wrong with that itself