Quite a few modern cars have "smart" alternators which reduce the voltage according to throttle position. The traditional smart relay for caravan wiring isn't nearly smart enough and either cycles on/off at low frequency or chatters at higher frequency. Smart caravan relays can be adjusted but not low enough to avoid the problem, and if they could they'd stay on when the engine is switched off !

I resolved the problem by having a Durite 140A VSR fitted which styays latched once triggered.

It's ludicrous that towbar fitters haven't yet woken up to the problem, it's been around nearly 2 decades on some models.