It actually makes more sense with no battery charging function. The PO either wired the converter backwards or rewired the inside backwards. Having no battery charger function means it would have appeared to work (lights and other non-electronic stuff) and wouldn't discharge the battery when plugged in.
If they wired it backwards, the loads and the battery plus would go to the large lug (white in the schematic), battery minus to the red wire, ground to the fuses (blue wire in the schematic). A picture of the wiring may help us if it is this.
If it's been rewired internally pics of the insides of the converter would help us also. Try to get as much detail of components and wiring as possible, maybe one overall shot and several detailing the wiring and components. I'm particularly interested in the wires coming out of the transformer going to the diodes (Diode1 and Diode2 in the schematic), and how the other side of the diodes are connected, these are what I think are backwards. It looks like the mfr didn't use a circuit board so it should be very easy to unwire the diodes and reverse them.
I'm curious, how does the battery get charged while plugged in? Is there another converter somewhere?
Stewart, Brenda and kids
2008 Newmar Canyon Star 3410