I deliberately let myself get snowed in at 11,000' in my Jeep during one September outing. Was actually a controlled experiment for me.

Had an air mattress, 2 foam pads, 2 sleeping bags and lots of warm clothes.

Cracked the windows for a little ventilation and waited out 2 snowy days . Final night was below zero because the clouds cleared around midnight.

Ice on the outside of the jeep from the snow. Ice on the inside from the condensation. Had to wait for the sunshine to warm things up before I could see out.

One of the bags I used is riding in my newest Jeep as my just-in-case bag. But it was cold at 35F when I bought it in 1983 :-)
I like to have 2 bags when actually camping. More flexible and redundancy is a good thing for assured sleep.