Its not mysterious or complicated to get a leisure battery system up and running on your van. You don't actually need a zig system and the only requirement is to get charge to the battery when the engine is running, but not allow the vehicle battery to discharge into the camper lights etc. This obviously means a totally separate 12 volt system for all the camper lights, pumps etc. In its most basic set up, you could just wire all the lights and camper stuff to the leisure battery via fuses or even one fuse near the battery. 10 amp should be enough. Please put the fuse as close to the battery as possible though so any wire from it is protected. Its not a nice site to see a short across a battery. Obviously the neg of the battery goes to the body of the van. Make it a nice solid connection onto the body. Scrap old paint away for a good connection.
So that is the basic camper electrics sorted out. You can of course use switches to isolate certain things, like a separate switch for the water pump and another for the lights, but its not really necessary. This is really all the zig wall unit does. Essentially a fuse board. It does also control charging the batteries from the mains supply, but that comes later as a luxury extra.
Now the critical bit. What they call spit charging. In essence, all this is, is a system to connect the leisure battery to the vehicle 12 volt supply when the engine is running so it can get charged, but to disconnect it when the engine is stopped so both batteries are not powering the camper lights etc as you don't want a flat vehicle battery in the morning. This could be as simple as a wire from the positive of the vehicle battery to the positive of the leisure battery with a switch to break and make the connection. Of course you would need a good memory to operate the switch every time you start the engine if you want to charge the leisure battery too and switch it off after if you will be using the camper lights etc.
It would be better if this happened automatically. Luckily, its very simple to do by using the alternator small wire (the one that goes to the red light on the dash) and connecting it to a relay. Get a 10 or 20 amp relay from a car shop and make a connection to the small wire on the alternator. You can tell if you have the right one as disconnecting it will mean the red light will not come on, even with the ignition on and engine stopped. Take a connection from there and connect it to one of the relay connections for its coil. The electromagnet inside that makes it flick the contacts over. Connect the other side of the coil to the body of the van or convenient negative. Its all low current on this side so can be thin wires.
If all is well, the relay will stay off when you turn the ignition key, but it will click over the contacts when you start the engine and the alternator produces power.
Now the next step is to take a wire from the positive of the leisure battery to the positive of the vehicle battery, but it must go via the relay contacts. Any contacts will do. Just make sure they are closed when the engine is running. These wires need to be thick as they will be carrying around 20 amps at times. Also its vital to put a fuse at each battery connection. I use inline fuse holders with a 30 amp fuse. Its vital to do this to prevent any risk of a short to the body due to the wire insulation scaffing and touching the van body. Without the fuses, it will mean lots of smoke from melting plastic insulation and possibly a fire. Very unlikely, but not a risk worth taking.
So all being well, you should now get charge from the alternator to the leisure battery when the engine is running. Check this by putting a multimeter across the leisure battery terminals and starting the van. The volts should creep up as it gets charge. As a second test with the engine off, put the meter across the vehicle battery and then go and put on all the lights and everything you have in the camper section. Make sure the vehicle battery voltage does not drop a fraction. A digital meter might be good for this test (Maplin do them for £4.99).
So now you have a basic split charge system with the camper and van 12 volt supplies isolated from each other, but allowing charging of the leisure battery from the alternator when available.
You can also have mains charging of the batteries, but this is when its probably easier to fit a Zig control panel and mains charger.
Hope I haven't missed anything here. I'll edit it later as rushed it off before my tea.