Something I thought of doing for heat if we lived aboard another winter was to buy a propane RV furnace (hopefully used) and just mount it on the deck somewhere since the deck is weather tight under the shrink wrap. I was then going to run flexible ducts along the deck to the different opening ports. Then, when spring hits, just take the furnace off of the deck and maybe even re-sell it.
I also used the bubble wrap type insulation in the lockers, but nothing on the rest of the "walls". I rolled out regular fiberglass insulation (the pink stuff) onto the deck and cabin top to help keep things warm. This made a big difference, and REALLY cut down on condensation. One of the electric heaters we used was a hot oil type radiator heater. This really cranked out the heat, but to be effective, there had to be some motion in the cabin to circulate the warm air. Some mornings were a little chilly until we got moving and started the air circulating.
Catalina 38, Hull #112
Grosse Ile, Michigan