What kind of electric heater are you using? I suspect it is a radiant heater? I lived aboard my old Ericson 30 for a few winters in Washington DC and Annapolis and noticed that my friends that used radiant heaters all had a problem with condensation. I used two forced air heaters that blew hot air and circulated it around the boat. Both were on the floor, one aft below the companionway ladder blowing forward, and one as far forward below the v berth blowing aft. The warm air circulating through the boat dries the air (humidity goes down as the air temp goes up), and helps to keep the condensation from forming on the interior surfaces. I also put aluminum foil over the port holes to act as a strom window for insulation, which stopped condensation from forming on the glass. This setup worked in temperatures well below freezing and with snow on the deck at times. Hope it helps with your problem.