Sounds like battery... If your battery was low, why? Run down or drained from non-use? You say your battery is now "good"....how did you test? Voltmeter or load test? A battery can read "good" with a voltmeter (>12 volts), but, still have a dead/shorted cell that will prevent amp flow, even though it reads a static volt level of 12 volts or better.
My recommendation would be to temporarily swap your trailer's battery for a "known" good one (out of your tow vehicle, for instance) to eliminate that as a cause. The only other "device" that would cause those symptoms (in my experience) would be your converter is failing...or for the really simple; a corroded ground connection
I've seen a loose/corroded ground connection give those symptoms and if your battery is, indeed, good; I'd start looking at connections from the battery back first, as that is cheaper and easier than a converter! Bad connections will also keep your converter from charging your battery (maybe the reason your battery went low?). Hard to tell without a lot more information.
Maybe somebody else has some ideas...