Checking that the lights all work is the last job before setting off - I stand at the back of the van, while my husband operates each set of lights in turn. If anything fails to work, it is investigated and corrected before we leave. And we always carry a full set of spare bulbs, both for car and caravan.
A couple of years ago, we had the misfortune to be following a large, newish caravan and tow vehicle along the M6. After it had changed lanes several times, without apparently signalling, we came to the conclusion that none of its lights were working. After witnessing a very close thing on the last occasion that it pulled without warning into the middle lane, my husband, against my advice, pulled out and accelerated above the 60mph limit (we were towing, too, at the time) and as we passed, I wound the window down and tried, with words and gestures, to warn them of the danger. A few miles further along, we came to a service area, and pulled in, and expected the faulty outfit, by then just behind us, to pull in too, at the very least to investigate. Needless to say, they did nothing of the kind, but sped blithely on, presumably to leave yet more road-users cursing caravans.
People who do not take even such elementary precautions do none of us any favours. No wonder caravanners have a bad reputation, when we ourselves witness behavious like this.