Two items still were not yet installed: the 12V power supply from an external 240V mains power, and the recharging of the leisure battery from external 240V mains. I had already made the plans and wire diagrams, I only needed to make the actual connections.
For the battery charger I used one simple toggle switch to trigger two relays which would reroute the connection of the leisure battery: in normal condition the battery would be connected to the whole electric / solar controller setup; when activated, the relays would interrupt this connection (both the +12V and the ground) and would connect it to the output of a 12V car battery charger, which is connected to the 240V mains. A green LED indicates that the battery is being charged.
For getting 12V power while the leisure battery is being charged, it is advised not to draw current while the battery is being charged. The charger only provides up to 4A in slow-charging mode, and it would be overwhelmed to also drive all the consumers, if the battery is already quite flat. Therefore, the power for all these 12V consumers must come from another source: a 240V-to-12V converter was needed. When I checked online, I noticed that transformators and adaptors were either very expensive or did not provide sufficient power. I wanted to be able to have 30A at 12V, which is 360W. The solution seem to be switchable power supplies. And fortunately there is a good source for low-cost high-power 12V supplies available: PC power supplies. They provide high power, stabilised. I got an 800W power supply, which then theoretically could provide 66A. There are many web sites which show how to connect the wires. Important is that a power-on lead needs to be connected by a switch to Ground - this then powers up the power supply. All yellow cables need to be connected for the +12V line, all black cables together form the Ground. There are also +5V lines (all red cables) which I am not using now; and there is also +3.3V. But I only used the +12V and teh Ground. The Ground is steadily connected to the consumer circuits' ground, and the +12V is switched by a relay which connects the consumer circuits either to the solar controller output or to this power supply.
Works like a charm! Now I can go to any campsite with an external power supply and recharge the battery and at the same time still operate all my 12V devices (light, TV, refrigerator).
The power supply and the battery charger are plugged in:
The charger is charging the battery:
The line tester shows that everything in the mains line is properly wired (left picture), and the whole 240V setup is fully functional (right picture):