The fact that your camper's accessories work when your Converter (Charger) is plugged into 110-VAC tells me that your camper' electrical system is OK.
So, it then comes down to your battery and why it is not providing 12-VDC without the Converter?
The first thing you should do is check for loose connections at the battery.
Place a meter across the battery terminals while it is hooked up. If the Converter is working and the battery is low then the voltage should be around 13.5 VDC, if the battery is not charged, then the voltage should be around 14.5 VDC. which would indicate a battery problem. By the way, you did check the water level in the battery, didn't you?