Is the battery cable disconnected? Or just the battery disconnect switch? Have you physically checked to make sure the solar panel is supplying power to the battery? How powerful is the solar panel? 10W? More or less? Some of those solar chargers are basically just trickle chargers, and if your battery is a little old, or not in the best of shape, they won't keep it charged.
Honestly it sounds like the battery is still connected to the trailer, even though the battery disconnect switch is turned to off, and it may be drawing more power than the solar charger is providing.