The key is buying a DC-DC converter for the CPAP machine. That is the most efficient way to do it. I bought mine of of Amazon.
KFD DC Adapter Car Charger for Resmed S10 Series,ResMed Airsense 10 S10 AirCurve 10 Series CPAP and BiPAP Machine,90W Resmed S10 370001 37297 Rockpals 300W/500W...
I have a LiFePO4 battery powered one that worked great at Round up. I have a folding solar panel that topped it off every day.
Somikon Portable Power Station, 298Wh LiFePO4 Battery Backup, PD 60W Type-C Quick Charge, 300W Pure Sine Wave AC Outlet Solar Generator Power Supply for Outdoor...