Not to steal the thread or anything...but I did have trouble in near
freezing weather last year, coming downhill from a campsite in the Sierras
in the morning before the engine warmed up. It just didn't get warm, no heat
from the heaters, nada. I checked the thermostat once home and it stayed
closed up to around 195-200* F, but replaced it anyway as I had a new one
sitting there.

I think it's this combination of factors--downhill with no load on the
engine for many miles, 35* weather, starting cold with little warmup
time--that did it.

Engine ran crappy and we froze.

Now back to our originally scheduled programming.