HI,
I have had a CPAP for about a year, and at first (for about 3 days) I used the humidifier, but have not since that time. I do not miss the humidifier, as it provided to much moisture.
I would suggest a #10 wire from your battery to where the CPAP will plug in. A friend had one and we installed 4 batteries in his large fifth wheel, along with 400 watts of solar panels to allow him to dry camp. He found that he still needed to run a generator every once in a while, because of other energy needs (running a laptop, or watching TV and lots of lights, ect).
A generator or a couple of solar panels will really help your battery power. Just a basic load on a RV is 35 AH daily to run the CO meter, propane detector and refrigerator, about what one of my 120 watt panels can put out on a sunny day.
SunElec.com is a good place to look for solar panels. I would suggest them in the 100 - 150 watt size, anything larger will be to large to fit on the roof.
I have a pair of 120 watt, and pair of 45 watt, and a 75 watt panel. They are arranged around the various vents, and avoid the area near the TV antenna, it will shade the panels.
Good luck!
Fred.