Ah, we're Bristol and have an ex mobile youth centre that we've adapted for our boy who's in a chair. You could have had a look for ideas.... I don't think we're ready to sell it yet but thought you might want a look.
Bottom left is showing a flame fault I believe. Maybe turn that off then it might use 240V. Long shot..
Then, it's checking there is actually 240V at the unit, then has the thermostat tripped or is the element dead.
You could just use diodes after all of the switches. The small voltage drop might actually be beneficial to your lights. I.e. they say 12V but you'll have c.14v.
Switch A to anode of one diode.
Switch B to anode of another diode.
Join the cathodes and wire to light.
It sounds like the split charge relay and the fridge relay are fed from a common point, but this isn't actually connected to the battery. So you have a link from the leisure battery to the fridge 12V.
What is the current draw when the van is just sat with all of the systems on but not being used? PLC/inverter/router etc. All that stuff, what does it add up to?