Please help me - how to use camper plug sockets?

[ginkster]

But if the van can start (ie not dead starter battery) then why does the inverter not work if its wired to the starter battery?

Not sure, but could the SB voltage be below that required for the inverter to supply 240v (enough for the lights to come on though) but just enough to crank the engine? The OP did mention getting low SB warnings earlier in the thread. Either way, I’m intrigued where the chunky black cable from the SB + terminal goes if not to the inverter!