Skidayjoo, first of all, check the battery wiring as Fhenn says. Next, do you have 120vac inside the trailer? Do the outlets work? You need that for power to the converter. Let's make sure you're getting power from the house, through all the GFCI outlets, into the trailer.
Then check the status of the converter. The 120 VAC in comes through a circuit breaker, and the 12VDC comes out through a fuse. You'll need a voltmeter or test lamp to see if the output is leaving the converter. There may be an in-line circuit breaker on it's path to the battery. Often a small box with two terminals, in line near the battery.
Marty and Martha
2015 Z-1 211RD
2012 Nissan Titan