Check the floor level first next time, saves weeks.

Man, that pump sack trick is a game changer. I had the same thing happen with my first pad and could not figure out why it kept going flat on cold nights until someone told me the same thing about breath moisture. For your older pad, the first thing I would do is fill up a bathtub or sink with water and dunk the whole inflated pad in there. Any leak will show up as bubbles, even a tiny pinhole. Check the valve area first because that is where most leaks hide, and sometimes it is just a dirty valve that needs a rinse and a little silicone lube. If it holds air fine in the tub but still goes flat in the cold, it might just be the temp drop making the air inside shrink, which is normal and not a leak at all. If it is a slow leak at a seam or a baffle, that is usually not worth fixing on an older pad and I would just use it as a loaner.