Good for you for using the boat off season.. For the occasional cool night like that you could use a
hatch tent or throw a sailbag over the
hatch.. If you have a
hatch tent you could leave the
hatch cracked too, improving ventilation.
You two are the major source of moisture in a closed up V berth on a cold night. Not breathing doesn't seem like a good option
so preventing the hatch lens and frame from getting chilled will help, as will not closing the forward compartment up tight.