One major difference of how I keep the boat is that in the off season it is in a slip, connected to shore power with a temp regulated heater in the cabin connected to the boats electrical system.
I have that problem long time ago
If you use shore power in winter on blocks with no earth
You need to earth boat with 60cm brass bar hit it in the ground below boat and earth shore power from boat to this
Other thing you can do is put clamp on prop-shaft with earth during storage
Hope this Will solve you're problem
Thanks for the suggestion but the boat is actually in the water in a slip - not on land on blocks.