I had two 12V deep cycle batteries and solar on my Winnebago View. I was able to run my CPAP and never had a problem with a battery running dry.
My new 1987 Sunrader has only one 12V coach battery.
I have seen external power supplies for CPAP's and am considering this since I see no place to add more battery capacity, the single 12V sits in the engine compartment and there is no room for 2 6V deep cycle batteries.
Anyone used one of these external 12V power supplies? http://www.cpap.com/productpage/bps-freedom-travel-cpap-battery-kit.html
Iflyfishwhenmycpapallowsmeanightsleep