Winter's almost here: ok to add antifreeze to black water tank?
Iím not sure my expertise is sufficient to effectively answer this question - but Iíll attempt something for your consideration and trust others will have better advice to follow....
1) How full is your tank? The freeze issue is expansion so for starters, unless youíre at 85+% capacity, I donít think the tank is likely to crack.
Adding a gallon jug or two of AF (again, depending on how full it is) might give you some peace of mind and shouldnít hurt anything - when you winterize and run AF through the toilet flusher, some makes it down in to the tank to get flushed out when dewinterizing next season.
2) below freezing for 3 nights - how far below and is it only for a few hours at night and back above freezing during the day? I think if youíre getting down to say 30 for 6 hours overnight and then above 35 during the day, youíre not likely to have much of a problem.
3) are you staying in the trailer during the freezing temps? If so, depending on year/model, running the heater will ensure some hot air gets to the rest of your pipes which is what Iíd worry about more than an unfilled black tank.
Having said all this - whatís the reason you canít just dump the tank? If youíre near a campground or a Cabellas or even some highway rest areas so equipped, you might be able to make a quick trip and ease your mind a bit. If youíre staying at a house that has a clean-out for the sewer, thatís a possibility too.
Just some thoughts and suggestions. Sadly, this free advice is worth everything youíve paid for it so take with a grain of salt and see if more experienced folks correct anything Iíve said here.