Regarding pure sine wave or modified sine wave inverters. It depends on the brand. My first and current Respironics CPAPs are okay with a 300 watt modified sine wave unit.
The last time I checked, the Resmed web site said a pure sine inverter was required for their CPAPs.
I don't know about other brands. Their customer service could probably tell you.
In my motor home when shore power was not available I used a 300 watt MSW inverter because it was difficult to run the dc to the bedroom slide and very easy to plug the inverter in by the bed. If I had a need to run the machine often without shore power I would have installed the dc wiring. We very rarely were without power so I didn't bother.