Chances are, even if you had the battery disconnect switch in the disconnect position, turned it to connect and infrequently charged it with the truck, the batteries will be discharged quite a bit. It may take the converter many hours to charge the batteries back to full.
Those idiot light meters (if that's what you have) are known to be inaccurate. Check voltage with your meter. That will also tell you if the converter is supplying voltage to the batteries.
You do know that if the batteries are left connected to the trailer system, the parasitic draws of devices inside will discharge the batteries within a week or two?