First off, welcome to iRV2. Glad you registered and posted your question.
My previous rig had a BIRD module, which as I'm sure you know is designed to permit charging of a dual battery system where both batteries have separate charging sources. When either of the charging sources is providing sufficient charging voltage, the battery banks are connected and charge, however when the charging voltage is reduced or eliminated, the batterys isolate to prevent discharging of the chassis batteries.
Several years ago, I had some questions regarding the operation of my unit on a previous rig and spoke with the folks at Intellitec. I can tell you, their technical service department was sharp and answered all my questions and walked me through some troubleshooting steps. I'd suggest calling them at 1-800-251-2408 and explaining your problem.
As other helpful information, check out the Technical Service Manuals
for all their products including the BIRD modual.
Here also is the direct link for the BIRD Modual service manual
The service manual identifies the defect symptom of chassis battery discharging into coach. Again though, if the manual doesn't give a good enough diagnostic method, give em a call.
Good luck with the troubleshooting.
Joe & Shelly, Justin, Tyler, Alyssa | Butler PA
2008 Tiffin Allegro Bus 43QRP|Cummins 425|Honda CRV