Hi Hacky, sorry I didn't get to see that post of yours until today. What you are describing sounds like what used to be called a "core balance relay".
I haven't had much to do with caravans other than to know that they have to have double pole switches and power points, that is to make sure that in the case of the lead connecting them to the mains is wired wrongly that both the active and neutral are switched off.
I run RDC's in my house and shed but I do have an earth rod and the Neutral of the inverter is earthed at the switchboards to make them work.
Older inverters would not allow the neutral to be earthed but all new complying ones do.
Also some of the older switch mode inverters also had a floating output that meant there was up to 110 volts on the neutral conductor. They were mongrels, I had an instance where a man was testing his batteries with a hydrometer and was getting small shocks off the batteries as it was raining at the time.
We fixed that by earthing the negative on the battery bank.
I will experiment for you and get back. It may be possible to get an RCD to work by earthing the neutral at the inverter or switchboard and connecting the earth wire to the earth on the inverter. I will check it out and get back to you.