All you need to do for that black tank flush line is blow it out with air. Get a camco blow out plug for a couple of dollars, it will screw into both the black tank flush and city water connection. Set your compressor to 40psi and blow some air down that line. You will hear the water that was trapped in that line spray into the black tank. Make sure there is a decent amount of antifreeze in your black tank and you're good to go. I did this the other night and it took less than 3 minutes.