Right. I've spent many hunting days in a tent, several on Kodiak Island. In each case I ran a 12v car battery. As a rule, I pick the biggest (most amp hours or reserve minutes) battery I can find that fits my guide's truck/SUV. I'm not packing it on a plane so why not leave it as part of a tip?
I have a PDF somewhere that talks about this in great detail. The numbers you need are the number of Amp Hours the battery will provide, cross referenced against the number of amp hours the CPAP pulls.
As a general guideline: Reserve Minutes / 2.4 = amp hours. So a battery with 100 reserve minutes would have 41.66 amp hours until the battery dropped below 12v. If the CPAP you are using draws 1 amp per hour, then it would last about 42 hours...or five eight hour nights of sleep.
I found that my CPAP burns .85 amps per hours without the humidifier (*that's the big amp draw). So when I travel for hunting purposes I leave the humidifier at home. I try to find a 120+ minute battery (converts to roughly 7.25 days). More than enough for a hunting trip in general, even on Kodiak Island.
Here is a great read on the subject.
http://www.resmed.com/us/dam/documents/articles/198103_battery-guide_glo_eng.pdf