The temperature at which water with "stuff" in it will freeze declines as a function of the percentage of "stuff". Your black tank contents probably wouldn't freeze until about 24 degrees, but why would you leave that stuff in there? Grey tank would probalby be OK down to about 28 degrees, but again, why would you leave stuff in there?
If both tanks are at relatively low content levels (say less than a quarter full) there's enough air space for them to freeze without doing any damage.
Frank and Eileen
No longer RVers or FR owners