I found it! It was an inline circuit breaker.
I was going to remove the batteries and test them but it's been raining all day so I worked in the camper to try to isolate the issue.
The bathroom is a fiberglass shell, the sink is part of the same shell but there is a waterproof door under the sink where there is a shelf and the toilet paper roll. I have an orthoscope so I put the camera in where I couldn't see or reach and saw it. I had to pull off the waterproof door and remove the shelf to get access to it.
I ended up just putting both leads on the same post with a dab of conductive grease. When I build my LifePo4 battery I'll install a better breaker and in a place that makes sense.
Thanks for all the help you guys.
I'm shaking my head wondering why someone would put a breaker in such an inaccessible spot.