How are you determining that the battery is charging via the tow vehicle and not by the converter?
Sounds like the converter is a reasonable place to start trouble shooting. The converter could be unplugged, have it's power supply breaker switched off, or have blow fuses rather than having gone bad.
You could also have a bad connection between the converter and batteries, or at the battery terminals themselves, so the converter is essentially not seeing the battery. Once you plug into the tow vehicle, the battery voltage display will see and report the vehicle voltage while your camper battery is still not in the circuit (and not charging).
Get out a multimeter and check voltage at many locations. Your converter should be providing around 13.6V to the batteries depending on what model it is.
2012 Aspen Trail 2710BH | 280 watts of solar on the roof | 2x6V GC batteries | 100% LED lighting | 1500 & 300 watt PSW inverters | so far strictly boondocking