I agree with you that it's wrong to not have some sort of wiring diagram for us and RV repair people to use when troubleshooting.
Donno the answer. My new trailer came with a worthless "owners manual". It was dated on the front cover 2 years older than my model and referenced things that were not on my trailer.
I guess it's more like a house. You would be VERY lucky to get any sort
of wiring diagram with a new home. Same for a trailer or coach.
Not saying it's ok or right, it's just the way it is.