Sounds like the battery is dead, or not charged. Can you try charging the battery with an independent battery charger, then hook it back up. Things should work, like the light.
Make sure you plug your camper into shore power. The on-board converter should charge your battery. Depending upon the system, it could take a while. So, if you're plugging in the trailer to electric power and then unplugging, that's not enough time to charge the battery. If the camper has been sitting for any length of time unplugged, most likely, the battery is simply not charged yet.