I think it's the battery disconnect switch being in the "disconnect" state, coupled with a dead or missing or bad battery.
Some background: The trailer has two separate electrical systems, one for the 120V stuff (air conditioner, microwave, outlets, TV, etc.) and one for the 12V stuff (lights, water pump, fridge controls, awning and slide outs, and so forth). There is a device built-in called the converter that takes 120V power and makes 12V power, to both charge the battery and power lights etc. when you're plugged in to outside power. There might also be an inverter to make (limited) 120V power from the 12V battery when 120V power is not available.
In the 12V system, there is a disconnect that goes between the battery and the main 12V distribuition fuse panel, controlled by a switch that's often near the entry door. Press the switch in one direction momentarily (it's spring-loaded to the center neutral position) and the battery is disconnected from the rest of the system, and press it the other way and it gets connected together. The converter is most often not on the battery side of this switch, but rather on the side with the 12V fuses, so when plugged into shore power the lights will work regardless of its setting. A few other things are connected to the battery side so that they will always work regardless of the switch setting, either because they need a lot of current (and the switch and fuse panel have a finite capacity), or because for safety's sake they should always be available. This always includes the breakaway brake system for the trailer, and often would also include the propane detector, slides, awning, entry step, leveling jacks, and so on--provided of course they are electric. Likewise an inverter, if present, is usually wired to the battery directly.
It sounds as though this switch may be in the disconnect state; but in any case, your battery is not charged and quite possibly no longer good due to being discharged all the way and sitting for awhile. As to why it was discharged, that could be partly because the disconnect switch was off, so the converter did not charge the battery when the trailer was plugged in, and possibly also something was using power from it at a decent rate or it was sitting for a decently long time. Check, for instance, that the breakaway switch for the trailer brakes is not pulled out and turned on.
Most likely there's not a complete wiring diagram available; most RV makers don't produce such documents, though there are exceptions (Airstream and Winnebago, if memory serves, and maybe a couple other makes had much more complete service literature readily available than most). That being said, many RVs are not vastly different from each other in the general way their wired.
