Probably not a good idea to even consider a 25' 12v extension. on a 12v line, voltage drop might be excessive on that length. But I see no reason not to use a good 110v extension cord. Here are the Power specs for the Dreamstation, as good 12v power point with a 15A fuse should work just fine. Just get a good power plug, not one of the cheap cigarette lighter types.
Electrical
AC Power Consumption (with 80W power supply): 100 – 240 VAC, 50/60 Hz, 2.0-1.0 A
Note: Power supply is part of the medical electrical equipment.
DC Power Consumption: 12 VDC, 6.67 A
Fuses: There are no user-replaceable fuses