Many of these machines actually run on 12 volts, as already noted.
But also, many have a high start up current
and so use a type of transformer that is pretty inefficient. The transformer packs have big capacitors in them to meet the start up demand.
If you cannot power the device directly from the 12v dc on the boat
I do suggest you find out exactly how much power the transformer is using. You may find it is much more than expected.
FWIW, I've powered my old Respironics machine straight from the boat's dc for nearly ten years now without trouble. About 16AH per night.