Hello ‘63-1566’ and welcome to the forum.
I can’t help you directly. The leisure electrics of all these vans is likely a bit different and probably non-standard. It’s certainly a long way off what is fitted now.
A first suggestion is to look at the wiring diagram that can be downloaded from the Hiacesoutheast website. It’s a bit generic and mostly covers van wiring, but you might find something related to leisure wiring and where it feeds into the main van wiring.
My LH119V has leisure electrics (but no factory-fitted inverter, though some do or did). I don’t fully know where this is fed into the van’s wiring but the positive (on mine) runs along the chassis rail under the driver’s seat. There’s a normal 80A Toyota fuse under there, which the previous owned had blown and neither he nor his garage could find. Instead he ran the leisure electrics from a non-standard mains hook up.
I’ve not traced that 80-A fused wire back, but am planning to. By modern standards the wiring is much too thin and you’d never fuse under a seat in an inaccessible location.
In order to investigate the leisure wiring a bit, the previous owner of my van got a wiring diagram for the camper set up from Japan. I’ve added a picture of it to the first post in my Show Us Yours! Section. It might be some use, but it clearly wasn’t for him.
Maybe some of the other parts of this first post might help you too.
Sorry not to be more help.
Good luck!!