I bought the sealed 12v DC battery that Respironics sells. It seemed pretty expensive, but comes in a very nice carrying bag with smart charger, charging cables, etc. There are probably other options that can do the same thing for less money.
I use it at home when the power goes off and I don't want to get up to start the generator. It's pretty heavy to pack around so I don't take it when flying, although as a piece of medical equipment, it does not count toward your baggage allowance.
I talked with Respironics to see if I can charge it while driving in the RV with my cheap non-sine wave inverter, and got different answers each time I called, so I'm not sure? As I say this, I need to check to see if my built-in inverter in the MH is a true sine-wave?