For me, I try to get it completely drained. I will flush and blow the lines to the toilet 1st, so no water/antifreeze mix will be in the tank. My the discharge line comes out the back middle of my black tank, so I jack up the front of the trailer to drain as much as possible. I will then drive around a bit, so hopefully any water left in there will drain to the discharge line. I will then drain that line.
Chap , DW Joy, and Fur Baby Sango
2006 Ford F150 Super Cab 4x4
2008 Surveyor 263