It would not be outside.
Gdetrailer gave you some good places to look. He is correct that some manufacturers use the circuit breaker he illustrated as a convenient terminal block.
Start at the positive battery terminal and work your way back, following each wire. You should find that breaker. Follow any wires connected to it. Your emergency brake switch would be in this path as well.
There is a good chance a previous owner removed it. The electrical tape wrapped on the bundle in your last pic doesn't look factory installed
Look at the bright side, you're learning a lot about your trailer :)