I have stored inside heated for 4 seasons. Most facilities have alarms with back up heat in the event of a failure. The marina has millions of dollars of equipment that would be damaged if the heat was lost so they are well equipped to prevent this from happening. Before my boat goes into storage I do all yearly preventative maintenance. I confirm all batteries are fully charged, waste tank is pumped ,flushed , pumped again until the tank is just clear water. I do not drain the water system. I leave about a quarter tank of water and keep the lines with water. This helps reduce bacteria and crud from growing in the lines no oxygen. If you do not plan to visit the boat through out the winter remove the neg cables from the batteries. I go to the boat a couple times a month to do work so I leave them connected but remove all 24/7 fuses. When I go to the storage facility I plug the boat in to charge the batteries, run a small amount of water just to flush some water, and flush the head. In the spring the boat comes out of storage I flush the water system with fresh water, then sanitize it with bleach,change my water filter and we are ready for another season.
There are many different procedures and steps people do when storing in heated. From nothing to everything. What I have done has worked for me.
