Depends on the age of the van as the same thing happened to us in 2011 when we discovered while on holiday that the BIP had packed in and wasn't charging the battery (which runs the internal lights).
Can you check the battery voltage? If the battery charger is working, it should read 13.7 +/-0.1 volts. If below that, it definitely isn't.
The answer in our case was to buy a car battery charger which we plugged into the 240V socket to charge the battery while away from home. Once recharged, the lights worked fine.
The first sign that anything was amiss was when the lights started flickering.