Supply make and model of converter/charger for better suggestions.
So how does an RV 12 power system work?
There are many systems including lots of custom homemade systems.
A Dutchmen Voltage probably has a converter/charger. It uses 120 volts AC to produce 12 volts DC. The DC powers lights, water pump, and other things. It also charges batteries. It can probably power lights and stuff with or without a battery connected.
So, lights and stuff work, but battery does not charge. My best guess is battery is powering lights. Converter/charger is not working.
So my charger has two fuses in the 12 volt fuse panel. One is 30 amps. The other is 40 amps. If either is blown, batteries will not charge, and converter will not power light without the battery. Of course if there is a loose connector or broken wire power will not flow either. The size of your fuses may be larger.
There is also a branch circuit breaker on my main panel that supplies 120 volts to my converter/charger. Since the fan runs, this breaker is probably not the cause, but you could easily flip it "off" and then back "on".
I wish you good luck and happy trails ahead!