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My 2 cents: I had a similar problem with my 5th wheel after only 9 months of use. It was a corroded terminal connector on the red wires where the converter, tow vehicle charging line, and the battery come together. The terminal was corroded by water and salt spray. 5 minutes with a wire brush, a coat of dielectric grease, and I was back in business. It was not at all apparent looking at it from the "outside." I took the nut off and it was obvious that the terminals had lost contact with each other.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW