With the unit plugged in, and the switch (if equipped) closed, measure the voltage at the battery terminals. If it's not about 13.8 volts or higher, a converter malfunction is the likely culprit.
To troubleshoot the converter:
1. Disconnect AC shore power and the negative battery terminal.
2. Remove the converter from it's enclosure. But do not disconnect the electrical connections.
3. Check near the 12V output leads on the converter for any fuses. Check them visually and with your ohm meter to make sure they are not blown.
4. Carefully reconnect to AC shore Power.
5. Place your volt meter leads on the 12V output terminals and verify that there is DC voltage. If not, the Converter has malfunctioned and needs to be replaced.
6. If there is DC voltage at the converter, then you need to trace the wiring from the converter to the battery to find out where the disconnect is. Likely culprits are fuses, relays, open connections or cut wires.
If you are not comfortable doing these checks, have an electrician do them or take the unit to an RV service center for evaluation.