Thanks for all of the education! I see now that voltage and capacity under load are two different things. Fortunately, the vacation was not ruined at all -- we were able to patch things together long enough to last the whole week.
And this was by no means our first camping trip in cold weather -- we camp several times a year in sub-freezing weather (down to about 20 degrees or so). 34 degree nights are usually no problem at all.
time2roll, I suspect that you are right that if I had measured voltage during the "collapse" (while it was under load), it would have been at 10 or so. But by the time I measured it with my multi-meter the next morning (around 12 hours after the collapse), the weak battery's voltage had rebounded.
Mystery solved!
And the bottom line here is that although these same types of batteries have usually lasted me five years (three sets in fifteen years), I will probably schedule replacement for every four years.
Google Calendar has made this a much easier task than it used to be -- in the pre-Internet days, it was difficult to give yourself a reminder for a date several years in the future. The only reliable way was to put a sticker on the item in question -- "Replace at the end of 2024."
And then hope that the sticker would not fall off! ;)
