If nothing 12v is working, good possibility the battery is dead or a bad connection near battery or at battery terminals.
Use voltmeter and start at battery terminals look for 12v there first.
The ground will run from battery negative to frame near battery, just trace the wire, clean the ground connection at frame.
If battery is good keep your meter grounded at battery and probe the connections in second pic for power. At least one of the large red wires should be 12v power coming in then follow it
Also in pic 3 i see large scotch locks, those are bad about corroding and losing connection. When I worked for rv dealer i replaced every one I came across. Pop them open and look for corrosion and test for voltage with meter.
For the tail lights you can use a battery charger, ground negative lead to the frame and probe the 7 way plug with the positive lead, the diagram for the plug is below. When you probe trailer brakes and ground there will be a spark because ground should e a dead short and the brakes should pull 5-7 amps working properly.
The wiring on those trailers is not difficult, you can fix it without a diagram. Jump in there and get started.