You need to get the 12v
cigar plug cable for your CPAP machine from the supplier. Although most people use their machines at home, the device actually uses about 9 to 10 volts of DC current natively which is converted by the supplied 110 cable when in home use. You don't need to convert your 12v
supply to 110v and then back to 12v
needlessly. You just waste energy in the process.
In your AS, running just the CPAP, you can usually get two or more nights of battery time from your battery bank after the normal late dinner and pre-sleep reading time light use. Your machine's wattage can be read off the label or supplied to you by the manufacturer; most are quite low.
My AS came with a 12v cigar plug outlet in the bedroom and beside the dinette.