I plug up exhaust,intake and keep 250W pan heater on all winter. FWIW Two yrs ago when I left the boat outside and shore power pluged in, I believe I had a lightning strike that fried the shore power connector but maybe not and was lucky it did not catch on fire. Wire moves under load from heat like a water hose over time will loosen. Had a friend who's boat did catch on fire from a loose shore power connection so do check that its in good shape and the connectors tight since it can and does loosen over time. Even seen it happen at the main breaker fuse panel in homes. Always wondered why I always see my electrician bro tighten the connection so tight when installing main panels.