You already have ducts down to the wet bay, but if you're not using your furnace no warm air will get down there. Maybe pointing a small fan back into a heater duct will provide enough warm air flow from the cabin down into the wet bay?
Or if you're handy like Red Green, you could engineer an electric heater to discharge into the furnaces distribution plenum to better heat the whole coach as well as the wet bay on electric heat.
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