GregC--The cPAP manufacturer (ResMed) is quite happy to sell me a 12v to the required 24v convertor for $95.00. $65.00 on Amazon if I were a doctor. (I asked my brother, an MD, if he could order it. His office doesn't have an Amazon account.)
There are guys on ADVrider using a deep-charge battery 16-18 Ah, and finding power to spare in the morning.
I'm still in research mode; lots more questions than answers so far.
Rumor is that the ResMed website has a lot of information on running from batteries. Guess that's my next stop.
Keep the ideas coming, both for my use and perhaps others who use a cPAP machine and have a different approach than mine.
Thanks again, guys.