Wow, I've been fighting the exact same battle with my 2008 Cayman!!!!!!. I believe mine may be narrowed down to the BIRD. When on shore power, as I understand it, the is a pass through that charges that chassis battery as well. I'm still trouble shooting and wish I could be sure. Here is a great document from Newmar that helps explain how some of these devices, if you have them, work;
https://comnet2.newmarcorp.com/insta...ctrical12V.pdf
Shoot me your email and I can send this document to you if you can't connect via url. (tried to attach here as well). From what I can tell, either short or genset should charge both battery banks. f
Maybe we can solve this thing together. The wife and I are heading out on our full time adventure before year end. Maybe run into you.