The battery may not be connected to the converter. The converter should be connected to the battery to charge it. The battery then would power the lights and other 12 volt stuff.
Maybe the battery is only used for electric brakes and is not connected to the converter.
There will be a circuit breaker or fuse near the battery. Mine has a little 3 cm metal can with "in" and "out" terminals. It is supposed to reset itself when overload stops.
There is probably a fuse in the converter/charger. Mine has two for the battery, a 30 amp and a 40 amp. The 40 amp blows when the battery is hooked up backwards. The 30 amp blows when too much power goes between battery and charger.
Get a volt meter and trace the voltage from the battery. Converter/charger should provide 13.6 volts. Battery should supply 12.7 fully charged to 11.4 volts fully discharged.
It is good practice to always test repairs before paying for the service. By test, I mean, try to use 12 volt appliances using only the battery.
Technicians are always in a hurry and often do not test. The also have a natural tendency to assume that what they did always fixes the problem. Take it back and tell them they didn't fix the problem.
Note: Always store lead acid batteries fully charged.
I wish you good luck and happy trails ahead!