I have a ResMed S9 with humidifier. The ResMed web site indicated it was okay to use a MSW inverter with it while using the humidifier but warned not to use the humidifiers with older models while using MSW inverters. Check the web site and manual to make sure it okay to use your unit with MSW either with or without the humidifier.
Although i could use the inverter with my CPAP, my company insurance plan at the time paid for my ResMed 12v to 24v converter. It covered all medical equipment at 100% as long as the doctor said it was necessary. My newer company plan has a deductible and only pays 80% after deductible is met and I don't think they cover the converter.
If you don't want to pay for the converter talk directly to your medical insurance and see if it is covered with a doctor's order. In my case he wrote it as I prescription. I didn't have any problem with the insurance covering it. I talked to 2 different people with the insurance on different occasions. They assured me I had 100% coverage.
The problem I had was with the person who handled my account with the medical equipment provider. She kept telling me I wasn't covered and kept referring to what Medicare covered even though I'm not old enough for Medicare. She refused to put the claim inan was going to bill me for full cost. She also kept trying to Bill me as having 80% coverage for masks and other items as that is what she was used to but I had the premium plan. I finally had to get someone else in the office to handle it when I went in to see them in person. Insurance did pay for it.