This is simplified 30AMP TRAILER Wiring setup that might give you info on what does what...
Keep in mind the trailers are basically desgined to run off the battery when not at an electric camp ground.
Your lights and alot of other items are 12VDC for this reason.
12VDC DC is provided by a working 12VDC battery and is also provided by a 120VAC Converter which produces 12VDC when you are hooked up to shore power. When hooked up to shore power the converter also provides a charge to the connected 12VDC battery.
The Inverter is usually a customer add-on to be able to run some of your 120VAC trailer items directly from your 12VDC battery. The inverter draws alot of DC Current so several batteries need to installed to accommodate this...
Take a look at this pictorial and notice what is run by 120VAC and what is run by 12VDC. This will not solve your particular but may give you some more insight on what questions to ask...
All trailers are unique but this sort of gives an idea what to expect to see...
Roy Ken