Turn all the lights off and see if the fan is still running. Check the battery with a volt meter to see what the current voltage is.
Unplug the camper, wait 30 minutes or so then check the battery voltage again.
If it is at or above 12.5 volts you should be okay, but I would check with the converter plugged up too, to make sure there is not an issue. If your converter is doing it's job you should get a voltage reading of around 13.2v when it is in resting mode, and the fan should not be running. Anything higher than that after the converter has been plugged up for a while I would suspect a battery or converter problem. Start with the battery first. Have it charged and load tested.
you also need to check the water level in the battery, if it needs a bit use distilled water to top up the cells, recharge and check the voltage again.
Easier for me to show this than explain it...
Aaron