Hi ProfJohnL,
Many thanks for replying.
The caravan (Eriba) has been owned by my father in law for most of it's life and I'm fairly confident it hasn't been messed around with too much (especially the electrics - he wouldn't go near them).
However, there is evidence (a short cable with battery post connections on one end, rolled up in a cupboard) that there was a leisure battery inside the caravan at some point. There is no evidence of any charging system, especially one that was 'factory fitted'.
I like your 'alternative solution'. However, if I connect a leisure battery to the 12v 'distribution' in the caravan as it is at the moment, it would connect to the cars 12v system (when the caravan is plugged in to the car). I could install an 'isolation switch' to separate the two systems, but I know that I (or my wife) will soon forget to switch it, so I don't like that idea. Also, I think a split charging system thing is a bit overcomplicated for our needs?
So......, I'm planning to cut the wire to the cars permanent 12v and have a totally separate 12v system in the caravan. (Note, the fridge will still be connected to the cars ignition-switched 12v for towing in the usual way).
While the caravan is at home on the driveway and at a caravan site with EHU (we don't plan on staying at a site without an EHU), the battery charger will be constantly keeping the leisure battery charged up and so can use it for the pump/internal lights.
Sorry, I'm repeating all what you said, but I'm just thinking it all out as I'm writing. I think the only thing different/additional to what you wrote is my plan to cut the wire to the cars permanent 12v (to stop the two 12v system fighting/draining each other etc) . I don't see any problem with that. Do you???
The caravan has only a 12v pump (used very infrequently) and dual 12v/240 lights (2 bulbs in each light and completely independent cabling), so the 12v for the lights will be very infrequently used.
We have a whale submersible 881 pump, which I think (the caravan is in a million pieces as I'm currently welding the chassis and repairing the wooded floor) connects to a 'socket' on the outside of the caravan.
So yes, I like your simple solution for a small 'leisure battery' and cheapish charger, plugged in to the caravan 240v socket.
Many thanks for your help and sorry for the waffle.