I have looked high and low for that diagram but cant find it.
Probably lent it to someone and forgotten abowt it. that hacks me off. neva mynd.
From memory the setup was from altinator to the dual battery set up. the lead to the 2nd battery had another dual battery isolator in it to the 2nd and third batterys. IE in the trailer. Also the trailer had a 240v to 12 volt reducer to charge the 3rd battery in the trailer. That circut was taken off a caravan circut. The only thing i kant really remember waz how he worked out how to stop the 2nd isolater swithing when the third battery wasnt attached. (no trailer being towed).
Sadly I havn't gotten round to building the camper trailer yet so I never studied it fully.