Motorhome in winter, The black water tank or cistern

If you have the cistern, no problem , it is internal and in an environment sufficiently sheltered from frost. If you have a nautical tank, and obviously not insulated and heated ( otherwise it would be another no problem ), the only thing is to try to unload it as frequently as possible , perhaps immediately after the hottest hours. Before leaving, lubricate the guillotine with a silicone spray (it is often this that freezes, preventing it from opening). I have seen someone who used to heat them with a hair dryer to open blocked guillotines: I do not recommend it , excessive heat could deform them and then make them necessary to be replaced. If it is necessary to defrost, use only a de-icing spray , which is also useful for windows and locks.

Motorhome in winter, The black water tank or cistern

With a nautical tank you will have autonomy of at least 2 or 3 days , if this is sufficient for you, unload when you have gone down to a warmer area, the movement of the camper and the higher temperature will favor emptying. For needs of safe protection from freezing, for example for a journey in very very harsh climates, the solution for the marine black tank is to insulate and heat.