Not sure 100% with your rig, but I would guess it runs of the accessory fuse block on your engine (thus your starting battery).
Adding an accessory outlet wired (and fused) from your coach batteries would not be all that difficult. Make sure the socket is rated for the amp draw of your CPAP and fused accordingly. Red/black twin battery wire is very flexible and can be routed to your bedside area from one of your lighting fused DC circuits.
If you use the suggested socket below you will also get 2 USB charging outlets as well.
12 Volt socket
12 gauge hookup wire will carry 15 amps of 12 volt DC up to 30 feet.
Red 50 ft True 12 Gauge AWG Car Home Audio Speaker Wire Cable Spool BPES12 50 | eBay
12 gauge wire fuse holder for 15 amp fuse
Fuse Holder without fuse
FYI - The USB ports have a small current draw at all times due to the design. If you boondock and want to be able to switch off the 5 volt chargers I would install a switch next to the outlets or use an outlet without the USB chargers.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW