Can you put in temporary reflex insulation (reflective foil covered bubble wrap type) perhaps held in place by flexible battens or Velcro? Are you shrink-wrapped? if you are make sure you have lots of vents in place.
If you add a heater that burns fuel, make sure it is vented outside, that will prevent it from adding moisture, and more importantly carbon monoxide.
I saw a very nice small looking dehumidifier at Home Depot last summer, and thought it would be great for a boat.
It may have been this one:
25-Pint Dehumidifier-SG-DEH-25-4 at The Home Depot
looked nice and compact. I have no idea if it was any good.