My CPAP has a transformer that plugs into a 120v AC socket but outputs 12v DC to the actual machine. The company makes a 12v DC power cable that plugs into a cigarette lighter type outlet. I bought that cord so my machine will work even without 120v AC power.
But $^&_(*&) Airstream didn't put a single 12v DC cigarette type outlet in my trailer. None at all. I had to install one on the side of my bed myself. All is good now though. (While installing that one, I went ahead and put a 3 outlet unit inside my closet. I can plug in several things at once if needed, now. {sarcasm}Thanks Airstream for making this so easy. {/sarcasm}