All the numbers mentioned seem OK. The new converters are more carefully regulated NOT to over charge. The tow vehicle charge rate of 13.8 is good. My SWAG [scientific wild ass guess] is that the previous diagnosis is correct. Between the connections of the tow and the battery, there is a poor connection. I understand you have checked and cleaned. I think at the big plug , at either end, or trailer gound, or battery posts connections, the connections are poor.
I'm not familar with late Airstream trailer's wiring systems. Earlier trailers use fuses for ground, connections to battery both from tow vehicle and converter. Spray lots of WD40 on every connection and loosen and tighten every wire screw. On some occasions a poor connection may become hot or warm to the touch. While checking for a poor connection, having a digital volt meter connected within view would be a great asset. I love a good challenge. Come on over. It can't hide from a meter......Frank
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