Do you have a multimeter that you can use? Typically you can pick up a cheap meter at HarborFreight for about $8. A meter would make it much easier to see if there is power in the battery and where that power stops before you can use it in the trailer.
Check on the tongue of the trailer and follow the wires from the battery positive post to where it goes into the trailer. Is there a fuse or a small metal box (circuit breaker) on that wire? If so you need to see if the power is getting through it.
Check that the battery negative post is connected to the trailer frame.