Graham, Oh, I can see your problem now with modern day canbus systems used for lighting circuits, you even have to ensure the bulb if blown, when replaced, is canbus error free. I used to wire up trailer/caravan wiring, but that was back a few decades ago, LOL. it was a lot easier.
So this does mean relays, so you can duplicate your lighting to the trailer via volt free contacts from the relays, and use separate 12V supplies.
Looking at the internet you do need an interface relay box, I found a universal version by just googe-ing, which might do you, but you will have to look into it closer to be sure it is suitable.
Many thanks, yes that was what I was thinking. Like you I have wired many towbars over the years but retired just before the new systems came into being. I have seen this relay setup and I think that I will have to go down this route.