I'd say it's unlikely that your CPAP actually DRAWS 5 amps. Probably (like mine) it's RATED for 5 amps including the humidifier and in normal use draws much, much less depending on the pressure settings and ambient conditions. I bought a big honking 100 amp-hour AGM battery and an efficient pure-sine inverter to run mine when boondocking, because my vintage trailer has just one battery.
What I should have done instead of believing the specs is used my Kill-a-watt and actually MEASURED what the thing uses. I sized the battery thinking I might get 8-10 hours of run time out of it. I find that I can actually run it 3-4 consecutive nights on that battery before the inverter complains of low input voltage!
My CPAP is a Resmed S8 Compact with the humidifier, and I only need a fairly low pressure setting. If/when I get another unit, I'll look for one that will accept a 12v
DC input, but I find that with a good inverter I don't have any trouble with my current setup. I have a 600W Xantrex Prowatt SW600 and could have gotten by with a smaller inverter as well, if that's all I wanted to do with it.