A solution that we've used on boats and older RVs that didn't even have an inverter, was to tap into any 12v reading light at the head of your bed, and install a 12v outlet, like the old cigarette lighter style. Then just plug a basic inverter into it. If you look at your CPAP it should tell you how many watts it uses, so just be sure to get an inverter that can handle that wattage. Most units will use between 30-60 watts and since most plug-in interters are at least 100 watts, then you're in good shape.
Ironically, once you plug in your CPAP into a 120 volt outlet, it usually goes to a transformer that converts it to 12 volts before going to your CPAP. That means if you wanted to bypass all that stuff, then you could simply wire it directly to a 12 volt plug and then plug that into your cigarette lighter outlet.
