I don't have a macerator, but I concur with others that the safest thing is to pour straight RV antifreeze (-50F is typical) into your empty black tank (I don't recommend grey because I find it's harder to ensure that it's completely empty), and pump it out with the macerator. I assume we're talking about just riding out a short-term cold snap in the 20's, where the rest of your plumbing is probably safe in a heated trailer.
Anything colder, or if the trailer is not being heated, I would do a full winterization of the plumbing. I provided instructions in a topic a year ago, if you want to see how I do it. In fact, being that it's 30F right now and lows around 30F for the next three nights (and few degrees colder in the suburbs), I'm considering winterizing this weekend. Another camping season has come to an end.