We have a Reflex diesel heater, though it's never been used. I was frankly considering going with electric only, for simplicity, but based on responses so far we will try out the diesel heater. We'll still need an electric one for when we leave the boat for extended periods. Needless to say it only needs to keep the air at say 50F or so just to keep things from getting too cold.

We do plan to get an electric dehumidifier. Hopefully that will help with condensation, but perhaps the diesel heater will help even more.