That is weird. I agree with your analysis on the BIRD intermittently not turning off the Trombetta, but from my experience I would think that it would be the coach batteries that discharge to nothing while the chassis battery would stay at 11V. Even with the use/store in store, Unless I hit the 100 amp breaker in the battery box, my coach batteries quickly discharge while the chassis battery discharges much more slowly. I can still start the coach after several weeks or more not being plugged in. By that time, my coach batteries would be dead. Since it happens so infrequently, it is very difficult to diagnose. One diagnosis you could try would be to open the 100 amp breaker in the battery box and see what happens. If the chassis battery still discharges quickly down, then it would indicate a problem at that end. The only other thing that I could suggest is that you monitor the voltages in the coach and chassis batteries to see if the Trombetta opens to disconnect them at the correct voltage. Since it happens so infrequently, it might take a long time to see anything. However, if the Trombetta is disconnecting at something different than 12.6, that would indicate something wrong with the BIRD or Trombetta - most probably the BIRD. The BIRD in my Axis was wired incorrectly at the factory so the coach batteries did not get charged by the alternator. But I cannot see how that can be the case with your problem.