The air pump in a CPAP is not the "power consumer". It's the heating circuit for the humidifier. The humidifier can be turned off as can the heating element for the "hose temperature". With those two features off, the system will run on a "Shumacher jump starter/inverter/compressor" for 3 nights (about 20 hours).
There are other means to power the CPAP, buying a 12VDC adapter for the device, buying a "stand alone inverter" and using the trailer battery being the most popular.
Here is the Shumacher jump starter we use. It's available at Lowe's and many other stores.
https://www.lowes.com/pd/Schumacher-...ter/1000569111