In both our fiberglass trailers I have found black flacky residue after some initial use. I concluded it was some oil ,left over from the manufacturing process , that had been on the steel inside the combustion chamber that burned off with use.I used some air pressure to blow it out and that seemed to help return the water heater to normal. May not be your problem but might be worth a try.
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Eric (and Mary who is in no way responsible for anything stupid I post)
"Beware of false knowledge; it is more dangerous than ignorance." George Bernard Shaw
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