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.