Does the machine actually draw 5amps at 12volts? Or is that just what the power supply is capable of providing? Look for a label on the machine to find out what it actually draws. Could be the power supply is sized to handle extra attachments, which you may or may not have installed. Example: A heated humidifier. If you turn off the humidifier/heater, you will save power.

I don't know this for sure, but it's a theory: In order for the CPAP machine to maintain pressure, it has to pump harder to overcome leaks. So make sure you have a leak-free fit. There are different types of masks available. I believe the full face mask will probably leak the most (thus having the highest power requirements), a nasal mask will be less leaky, and even less leaky will be nasal pillows. Now, just how much a leaky fit will affect power consumption, I don't know. As I said - it's just my theory. If you get up to go to the bathroom, turn the machine off. I have a friend who brings his CPAP on camping trips (we're lazy and stay in a trailer with electricity!) He gets up to pee, takes the mask off, and leaves the thing running. It sounds like a leaf blower over there by his bed, pumping its little heart out in a futile battle to maintain pressure.