On my lance TC the heated enclosed holding tanks are made that way with nothing more that a plastic housing bubble that encloses both tanks. They run one heat run to it with flex duct and then put a vent that comes inside the camper so the air can circulate. It works great. Your furnace should have small plenum on it where you can connect small flex duct. Just a thought. Maybe doable. It would sure be great to be able to use your fresh water tank. Of course any type of rigid foam strapped out side that bubble would help also.
