Caravan Power Isolator Switch

I missed this when you first posted it, so here comes a little history for you. From what you have said it is clear that the electrics in your caravan are far from original.

The vans from the 2000s and earlier that had a switch like that did not come with batteries. Vans were not commonly supplied equipped with 12 V batteries until the 2010s. They generally did not come with 12 V power points or car radios built into them. Vans built in the 1970s through to the 1990s nearly all had two electric circuits for their lights, the light fittings had both 240 V and 12 V globes in them. The 12 V lights were supplied directly from the tugs battery (along with the fridge when they went from 2 way to 3 way models.) In those days our 12 V lighting was easily supplied from the car battery when you plugged the trailer plug into the tug.

In the 1990s manufacturers started putting 12 V lights into vans and no 240 V lamps. The lights were supplied from a transformer (ie 12V AC) or sometimes an 12 V DC power supply. Those vans had a switch like the one in your van so the 12 V supply from the mains power would not feed back to the tug battery or the fridge. I reckon your van would have been delivered without a battery or battery charger. One of the subsequent owners would have added those. We would not be able to guess how things would have been done when the 12 V power supply was upgraded. It's just as well you have some idea of how to chase thing out.

When I purchased a new van in 1997, I sat down with the licensed electrician working in the factory and explained what was required in 12 V systems in vans. Back in those days very few electricians had any idea of caravan 12 V power systems. When I mentioned that I would like to convert the exterior light to use a high efficiency fluoro light he mentioned that Hybralec had some new exterior lights with 2D tubes in them to trial. He rang them and as a result I had one of the first of those fitted to my van. Up until then, all 12 V lighting in vans was car type globes of QI globes. Things progressed slowly from then until the mid 2010s when nearly all the vans came fitted with a 12 V battery or provision to fit one.

When you rewire your van you may consider scrapping that switch. My later two vans have not had them fitted. If you do install an isolation switch, I suggest you just use a high current breaker and position it so you can remove all phantom currents from your battery. Just have your solar system connected to your battery when you isolate it.