I have for sale a converter which allows the S9 machine, which requires a 24v supply, to run from a vehicle 12v or 24v DC socket, or direct from a 12v battery. The converter can power a humidifier and heated tube as well as the CPAP machine, although that obviously depends on battery capacity.
It was bought direct from ResMed in 2016 for a motorhome voyage in NZ and has only been used for that, plus a couple of nights in a tent (with a standard car battery) and a handful of times in our own motorhome when no mains supply was available. It comes complete with instructions, 12v cig-lighter type plug, and adaptor lead with battery clips. The same thing is for sale on eBay for over £100, a new one from ResMed is £64 delivered. You can have mine for £30 (PayPal, inc postage) or £28 cash on collection - I am in West Sussex.