Ugh a real face palm moment for me
All this time I thought my two battery systems were isolated using a BIRD relay system. A couple years ago I even replaced the relay itself. I guess I've always been in such a hurry I never fully tested it.
Turns out I have an Intellitec 00-00629-120, which is just a "IRD"
My engine can charge my house, but not the other way around.
I found it because after having recently installed a solar panel, I have been piddling when I get a moment here or there, to add a basic panel volt meter to keep a loose eye on things....not as good as measuring amps in and out, but it'll be something at least. I was going to tap into the 'Bird Relay', and that's what got me to looking....
....Now I have to figure out if I want to fix that situation by getting "B" for my "IRD"