I would think whatever antifreeze you get in that black tank line will immediately drain right back out. That's its entire design being mounted up high with the valve.
I think our first camper with the black tank flush was around '07 or '08. I have never even attempted to winterize that line in all those years on several different campers.
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2019 Montana HC 310RE
2010 Wildcat 29RLBS
2014 Ford F-350 Lariat 6.7
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