I read your post that you checked the battery voltage "at the battery."
If not then you must.
There are two fuses (current limiters) between converter and the battery. If one of them opened, you would see converter/truck voltage all over the coach; but not AT the battery.
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