[sourdough]

Think thru it;

It worked fine last year; you disconnected the trailer for a year? months? The batteries died...but you say they are in excellent condition - if you disconnected the trailer a year/6 months ago and it has just sat, the batteries are an issue - with the way you say it is working the converter is a non issue

You either have a problem with the batteries (don't know how you know they are "good" but a year just sitting hooked to the trailer without support will kill them), or if the batteries are actually good, something happened to kill the feed from the converter to the batteries. You would know those kinds of things.