Everything is fine now, the batteries thawed out nicely and stayed warm enough with the portable heater on the floor to charge all night. I left for home with 85% charge today.
And now I have an analysis target for that annoying monster of data that I have in my trip logs.
I am going to troll through that data and see if the temperature of the batteries on departure is related to the range. Most people are complaining about a big drop in range in the cold, but not me. And as far as I can tell no one but me is preheating the car from shore power and thereby keeping the batteries really warm.
Of course, I just had the one glaring exception of last Saturday when the batteries got so cold that they stopped charging from the slow charger. (I suspect the fast charger would cause enough self heating of the batteries that they would not have stopped charging at 10 degrees F ambient temperature.)
I expect that the data will show the range hardly affected by the cold weather as long as the batteries are warm.