Good point, I missed where OP reconnected SP. Still under the weather I guess.
The electrical path from converter output to Chassis battery is exactly the same as to Coach battery (through coach disconnect latch relay in BCC), with the additional path through the closed isolation relay. So yes, the 15 sec drop out is probably the BCC sensing the coach battery voltage dropping to approx 12.6vdc for 15 seconds and it releases the isolation relay; disconnecting the chassis battery. So 16ACE27 you have the solution I think. Poor connection somewhere from coach batteries (check both ground and +) to where they connect onto the BCC terminals? A strange one to say the least.
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