One of our CPAP units has the (expensive) 12V brick option I bought for it, and to power the other CPAP we just use our small sine wave inverter. I think that both unheated CPAP units use less than 2 amps each when running - maybe even less than 1 1/2 amps.
I'm getting real lazy in that lately I've even been just idling our small motorhome's engine for an hour to quickly pump 30-40 amps back into the coach batteries ... not even running the built-in Onan or getting out the portable Honda to top up the batteries, but they're available just in case.