I'm new to this forum but a lifetime camper and 10 year cpap user. I also have a Resmed S9 and I purchased the 12/24 volt transformer for the S9
from cpap.com. Mine came with a high quality battery adapter that you can clip directly to the battery and run the wire under the front bed canvas. This will be much more efficient than running off of an inverter.
Leave your heater/humidifier at home and attach a standard hose
to the side port on the cpap unit. If you feel you need the humidifier function try one of these
between the hose and mask.