CTEK D250S DUAL - MPPT and dc/dc battery combo charger

Now I'm even more confused. If it's intended as a isolator as in charging 2 battery banks then every MH I've owned has one. Trailers on the other hand don't normally have 2 battery banks and hence don't need a isolator.

By isolator do they mean it disconnects the tow alternator from the trailer? That function is very common in the US. As a DIY project a $5 relay will do the job. And I'd guess that most factory tow packages supply 12V only when the engine is running.