Your trailer probably has the battery disconnect switch wired on the opposite side from what most are. What this means in English is the batteries won't charge if the switch isn't turned on, and if you exceed the capacity of the converter the lights will dim or go out when you turn them all on. The solenoid (relay) could also be bad. Listen for a "clunk" when you push the button on the disconnect switch.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.