With the batteries dsconnected, you should still maintain a charging voltage from the converter. You may be reading the voltage of a dead battery @ 5V.

 With no batteries attached to the positive cables, a basic converter will put out about 13.8 -14V DC. A multiple stage converter/charger like yours can charge anywhere from ~ 13.2V -~ 14.4 V DC depending on the batteries' condition. The more battery charge that is necessary, the higher the voltage that will be used. A maintenance voltage would be its lowest output.

I would check the terminals at the converter, checking also the converter case to ground wiring. With power off and the batteries disconnected, you can check the terminals for resistance and continuity. Make sure the attaching screws are tight, and no wiring insulation had slipped into the terminals, when the wiring was installed.

I would physically remove each wire and check the condition of each of the ends. With the converter on and dead or no batteries, you should read voltages in the high 13's and low 14Vs DC.

You should also check the AC power supply into which the converter is sourced. If it doesn't have 115-120V AC, it can't opperate properly. It may be the victim of a poor breaker or a bad connection.

If none of the converter fuses are blown, the voltage to the converter is acceptable and the output voltage remains at 5V, you may indeed, have a bad converter.