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What kind of fill in the sleeping bag, Anon?
If down, take time to 'air out' your sleeping bag in the morning sun to dry out any moisture from condensation in the bag. Over two or more nights it would add up in there and decrease the R value significantly.
Another useful piece of cold weather kit is a reflective tarp as a ground sheet, shiny side up. That will raise the temperature inside your tent a few degrees.
