Winterizing Black Tank?

I use a quickie flush in my grey and black tanks. I am 99.999999% sure I don't have any thing hanging on the walls or the bottom of the tank. over the year I have read about the proud owners of the flush kings and how good they are. if they are so good then why are the TT manufacturers putting quickie flushes in there TT;s and not flush kings? having said all that, I do put some pink in the black and the grey tank, probably about a qt., each. that is just for the seals to keep them moist. the grey tank gets it's share by way of the faucets and the "P" traps

I winterize with the pink stuff. However much pink stuff gets into the black tank while I am flushing out the toilet line and toilet sprayer, plus whatever overflows when I fill the P trap of the bathroom sink and flush the sink lines, which drains into the black tank, is all my black tank gets. Same with the gray tank. Whatever overflows from the kitchen P trap and the shower P trap is all the gray tank gets.

That said, what does damage is ice expanding that has filled a line, breaking the line. As long as you drained the tanks so they are not full, when they freeze, there is room for the ice to expand without doing damage. It is far more important to be sure there is SOME fluid, even water, to keep the seals from drying out. If you drain the fresh tank, black tank, gray tank, and water heater you are good for winterizing those particular items. It is the water lines that you need to worry about.

A few gallons of water left in the waste tanks will not cause you any problems. No freezing damage will occur as long as you don't start yanking on the valve handle while it's frozen solid. If it makes you feel better to spend money on extra antifreeze to throw in there, by all means do so.

The MAIN thing to remember is to NEVER allow the black tank to completely dry out. No matter how well you flush it, there are always some solids left behind and these will turn to "stone" and become nearly impossible to remove (and they accumulate).
Good luck / Skip

I put enough RV anti-freeze in black tank to cover the dump valve. Then when toilet valve is closed pour a little over the top of it. I pour enough down the sink traps, shower traps etc. so enough goes down to cover the grey water dump valve. The anti-freeze keeps the valves moist so they don't dry out.