Back in the days when I flew 737 [ 10000+ hours] we regularly did bleeds off take-offs ....Now what if I forgot to reconfigure the bleeds after take off and climbed to cruise altitude using APU bleed? ....So the APU decides to shut down at 30 something thousand , loss of hot bleed air , packs still operating with ambient ram air so cabin depressurises rapidly , passengers feel cold air , masks drop.
Sorry for topic drift, but ....on most aircraft, if you shut off the bleed air, the packs won't operate at all. The Pack Flow Control & Shutoff valves will be closed (i.e. no pack output airflow). Ram air is only for the heat exchangers and will not power the packs in any way.
With no airflow in the cabin and differential pressure dropping, the outflow valve/s will run towards closed. With lots of warm bodies in an insulated and almost sealed container, the temperature may not drop as much as you think (perhaps not at all)
Rgds.
NSEU