We just went a week boondocking in the Sierras. I could have simply plugged the AC adapter into an outlet using the inverter, but I thought I would try to be more efficient. So, I bought the 12v adapter for my wife's Resmed off Amazon and an extension cable for the cigarette plug. I have 2 12v marine batteries from Costco, a 135 watt solar panel on the roof and a portable 100 watt panel I can move around to optimize solar exposure.
We ran the humidifier on low (no heated hose). Batteries never got below 86 percent or so by morning after running the CPAP all night. We had a relatively small number of clouds that week and the batteries were replenished by the sun by lunch time each day. I had my generator with me but never had to fire it up.