OK, looks like we (mobil RV tech and myself) ran down the problem. So I have to eat crow and defer to a couple of you guys who called it - LOOSE BATTERY CONNECTION. Yep, the Alt is putting out what it should be, the converter on genny or shore power is putting out what it should, no apparent bad connections, EXCEPT the positive on the battery terminal, which we could move with our hands. While I could blame it on the pounding we've been taking on the trip up here (which has been insane), we were able to tighten all the battery connections, but the main chassis positive was in fact loose.
Lesson - every oil change check the battery connections as well, which I should be doing anyway when I check the water levels....
Al