Ended up stopping at CW in Longmont. "Sure", they said, "We can test the batteries for you. Our first opening is June 24th..."
What a waste of time.
Since the alternator is keeping everything going while underway (BTW, the gauge is reading 14v consistently), I'll just wait until we get home to address the problem. We are in the Black Hills for a week so I don't need to worry about it for awhile.
I did find my echo charger leads are reversed, so I'll swap those wires while I'm here - maybe that will get some life into the chassis bats.