I've seen several examples of rugged, portable home designed units featuring deep discharge batteries topped off with solar photo voltaic cells... the quality and power output of the panels is rapidly improving, but I'm not in the business, so can only offer a suggestion...

for emergency backup, one of the 2000W inverter generators like the Honda EU2000 with a propane/natural gas adapter would be high on my list if I had to rely on a CPAP...

good luck