Da house battery be connected to da chassis battery wid a relay
Da chassis battery be connected to da alternator and starter
You are fortunate. I have talked to several people who had their machines go dark.
I suggest deferring to ResMed or Respironics about disconnecting the machine. Inside the BiPAP or CPAP is a pathetically small amount of deviant voltage protection. The companies rest all their laurels inside the medical grade power brick.
Any problem is compounded when using a non medical booster for 24 volt operation. The toroid and caps = LC harmonics. Check with the machine manufacturer. Thank god I have transient voltage supplies and the knowledge how to construct circuits.