I blow mine out with air. The line runs from the flush port up to my sink area in the bathroom where there is an anti back flow valve. Then from there to the black tank. If it were to freeze and break the anti black flow valve it would make a big mess inside the trailer the next time you use the tank flush. You may not notice that water is running inside the trailer until it were to run out and into the basement area.
I do winterize the the black tank flush, you will need to hook a short piece of hose to the black tank flush input. Fill the hose with antifreeze. Then use 40psi to blow the antifreeze through the black tank flush into the holding tank. I don't chance it. You don't want that to freeze. Deputydog