I had the same issue on my 2016 Axis. At first I thought the chassis battery had just expired, but when the replacement was dead after a week and the emergency start didn’t work, I realized I had an issue with the BIRD and/or Thrombetta. I was planning to diagnose the issue and replace the offending component when I discovered this:
https://precisioncircuitsinc.com/pro...ation-manager/. I installed it last weekend (pretty easy) and then tested all three batteries before plugging it in. I left it plugged in overnight (about 16 hours) and retested. All three batteries had charged (I don’t have the before and after numbers in front of me at the moment) and were nearly equal. Just to be 110% sure I am testing some longer stints of keeping the coach plugged in, but so far all seems to be working perfectly. I have pics and can document the BIRD to BIM wiring (what I found on this and another forum weren’t 100% correct at least for my MH) if it would be helpful to anyone.