If you want the "best way" as per the title, the best way would be to run new wiring totally concealed inside interior partitions, via ceiling cavity, in cavities in cabinetry, etc. I've installed tons (exaggeration perhaps) of various wiring types all 100% concealed all over our TT - front to back. Takes some patience and forethought. But if you're not a handyman type may not be for you.
You'll want to connect to existing 120 or 12 volt wiring already present in the slide otherwise adding it to a slide isn't a simple project. I think most CPAP's are 12 volts?
Never, ever run a 120 volt extension cord under a mattress or carpet, in a wall, etc. They are not designed and rated for that and you *could* potentially end up with an overheated cord and a fire, esp. with a light 16 gauge one.
