Ditto to Paula's good advice. Airstreams are meant to be used which includes very regular use of the fresh water in the tank. We have become too enamored of full hookups, so folks are disregarding the standard operating procedures of keeping their tanks (fresh and waste) in tip-top operating shape.
Use it or lose it comes to mind.
The bleach treatments should also be pumped through all the plumbing lines as needed.
I would be careful here until the water is clearly OK again. Understanding the source cause of the black gunk, in a trailer which is only a couple of years old, must happen ASAP.
Good luck, and please let us know what you do.
PS Bob thanks for the reminder about the replacement fitting.