Javelin thank you for your answer.
The only unusual thing that the two events have in common is very discharged coach batteries. I have brought them back up to full charge this time with an outside charger. That might have been the issue. The two fuses are next to each other on the 12 volt distribution panel and are 30 amps each. I think the converter is 40 amps. I wonder if this difference is the issue.
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