You may have a switch to turn off the converter (OEM was known as a Univolt), to keep the loud HUMMMMM from driving you insane while trying to sleep. It could be simply turned off. Try plugging in the trailer, and turning on all switches while watching your battery condition meter. If it suddenly comes to life, you have found your problem.
If not, there is a 50 amp + fuse as well as a 50 amp - fuse (to ground). If either of these are blown, you will have no joy with the converter.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.