Are your dump valves also enclosed within the heated under belly? If not the grey water line dump valve will freeze. Black seems to take longer to freeze because it ferments and warmer but it also will freeze if exposed. If you have a wet bay use a light bulb in the bay. Like other posts only connect to take on water and connect sewer to dump. If you have a wet bay use your furnace heat higher to keep wet bay and under belly warm. Using electric heaters inside can be a problem because it is the furnace that heats the under belly.