Ok! I figured it out. ...And unfortunately, yes, it was mostly user error.
The previous owner, or previous previous owner, (who knows) wired in another outlet at a junction box in the right rear. This was in the form of a cable and plug running out of the back hatch...
Who knows why. I assumed it was the plug in for the camper since the original appears to not be connected (which it is stealthily
). So, essentially I believe I was running power into an outlet. That was being be converted somehow by the inverter into 240vac at the outlets.
Now, I am too tired to even contemplate what exactly was going on in that inverter, but I am alright with just having everything working and surprisingly nothing damaged. Now it's time to call it a night.
On the plus side, I've checked a lot of the connections, and hopefully removed the last bit of owner engineering.