Why not utilize a completely separate circuit for the winter heater?
heaters do not require a ground. As such you can run them on a two element conductor. I would think that you could get one or two high quality exterior rated 15A extension cord, and plug them into the shore power
. Then have one or two electric heaters installed on the boat. You would have to secure the extension cables
securely to make sure they don't knock over the headers when someone trips on them.
This should be the same as keeping your boat neutral by keeping your boat from using the ground, while at the same time providing you with the required heat in the winter.